I once filled an annual ISA allowance (I think it was more like £15k at the time) - I'd been saving into a bank savings account for all my working life and once I started to read up on the possibility of investing, I just opened one up and filled it right before the end of the financial year. It was less than half of my savings though. Given I held it for 2014-18 and it almost doubled in size, my main regret is that I didn't put the rest of my money in ASAP too :) but I didn't really have any specific goals for it to help me make my decision so I hedged my bets.
Every situation is going to be different. Really the key questions are:
- What is the purpose of this money – available for spending on e.g. travel; emergency fund in case of e.g. job loss; being saved for a house deposit; early retirement savings; traditional retirement savings, etc?
- Therefore, where would it make the most sense to hold this money – keep in the bank, move to an ISA, move to a SIPP, etc.
For me personally, if someone gave me £20k, I'd put £19k of it in my ISA and buy myself some furniture with the last grand, ha. But at this point in my life I doubt I'll ever end up with such a large pot of money just 'lying around' again - money available for long term savings gets whisked away as soon as my pay comes in each month.
I personally wouldn't hold back from putting it all in out of market timing concerns – if I was certain the money could be assigned to long term goals, I see little benefit in drip feeding it (as cerat0n1a said, this has been discussed at length, but can be summarised as 'the market goes up more than it goes down, so it's generally better to get your money in ASAP'). I might do so out of personal circumstances concerns, e.g. if I was worried about the stability of my job. But that's basically saying my emergency fund is too small for comfort, so I'd assign whatever money I felt was appropriate to stay in cash, and then invest the rest for my long term plans.