I made a recent post on the topic, now modified with a couple different thoughts since I'm still actively researching this possibility. I'm 39, single without kids and am a nurse, relatively new career I still somewhat enjoy. 60 to 65 % savings rate. I will have the option of loading all of a year's work into a full time three months or doing it part time through the year. I realize not everyone has a career that they still somewhat enjoy, or one that allows for this type of flexibility of employment.
This is essentially my plan, still a couple years away from execution. Save about 12X spending, after that, earn enough to live, but not to save unless I find a job I thoroughly enjoy working. I assume an average 3 percent real growth on any money saved over the long term. This will have a doubling effect in 25 years when I reach "normal retirement" age & I'm eligible for SSI. At which point I'll be sitting at 4 percent rule, plus SSI with a 25 % chance to live 25 more years. I've run the numbers on Firclac/Firesim & monte Carlos. Calculating 50 percent of SSI benefits, even with adding in more costs for medical, this will work at near 100%.
IMO this plan "safer" than full FIRE at 4 percent for the following reasons:
1) When the market DOES get hit really bad at one or multiple points in the 25 year period of continued income, the option exists to ramp up work in the short term to take advantage of low equity pricing. This, of course, will be totally optional, but if one feels like working quarter time until traditional retirement is too much for their frail old bones, she could do it. Plus everyone loves a deal!
2) Diversified assets along with diversity due to income. If a bad inflationary periods hits, income should rise to compensate. As a result, there is no need to have a significant amount of low yielding assets such as TIPS or gold (I know controversial) in a portfolio to guard against inflation. This should help increase returns.
3) Continued employment provides one security they will not become obsolete in a desired field. This is the often recited argument of a return to work situation for an early retiree, better to earn more now and be safe vs return to work for a lower wage later. At worst, I only have to explain a year "sabbatical" in between jobs or tell my PT employer I want more hours, easily done. If all all assets take a shit, I can always resume work full time at my current income levels (inflation adjusted) without missing a beat.
4) Avoid the risk of "overworking". The forum contributor named Sol regularly points out how often the 4% rules ends up making people work too many extra years for no reason (I do not want to put words in his mouth, but this seems to come more from his posts than anyone else). If, in fact, a semiretiree sees better than expected real returns of 3%, her stache will grow to FI levels more quickly. She can then choose to move into full retirement at any time. Had she worked to 4%SWR, subsequently gets great returns, she can never have her time back, she'll just die uber rich.
5) Sequence of return risk virtually eliminated. This is by far the greatest risk of FIRE failure. If there is a poor initial sequence of returns... great! See point #1. Now our semiretiree gets to chose to work a little more, maybe half time, and save more when the CAPE is below the current 26. This should provide better long term returns for new savings.
The semi retiree, in an approximent quarter time work scenario, isn't spending much more total time at work. Anticipated returns, savings rate and income can have a HUGE impact on this equation. However, in a scenario like the current one, with high CAPE (ie lower anticipated 10 year returns) and someone with a high savings rate this is true. This anomaly works as a result of the tax implications in earning less per year over a longer period. Less taxes offsets the loss of returns via a bigger stache earlier, this appears to amplify the higher ones income rises. In this example, a person with 12X spending and 3% real returns making 100K gross has the following. 75k net income (single, maxing a 401K), saving 50K. 12X spending is 300K. At 3% real return it takes 5 years to reach 4%SWR. 25 years of work at .28 of full time on average to make the 25K net = 7 years of full time work. This assumes the long term real returns remain below mean, which probably wont be the case. Higher returns down the road mean less income(ie working) is required. Worst case, a semi retiree trades a few years of total work time for the opportunity to enjoy freedom now, work when it's most convenient, along with the added "safety" listed above. Many people seem to work to a lower SWR (3-3.5 %) to have similar "safety" anyway, which offsets the extra time a early semi retiree works.
IMO this is an AWESOME deal for anyone who started working towards FI later in life. I absolutely request that people pick apart these arguments, as I want to see if I have flawed thinking.
Another option if OP is just super burned out, take a look at this post about sabbatical years. It may be a realistic alternative to long term part time work, it's a well reasoned argument.
https://livingafi.com/2015/08/04/taking-a-gap-year/