I received a lump sum in Aug 2015 (about 25% of my total portfolio at the time) and invested it in my balanced portfolio in one lump sum. The market went through a lot of volatility in the 2nd half of the year, and was generally down through Feb 2016. That hurt my 2015 returns, but by mid-2016 I was back above water.
In May of 2016 I also received another large lump sum, and again invested it all in one go. That either took a lot of guts or a lot of stupidity. Especially when the Brexit vote happened shortly after. But the market performance in 2nd half 2016 was strong, so the lump sum approach helped me. Despite Brexit, Trumpit, rising rates, and a lot of global uncertainty I am definitely ahead through lump sum investments.
Both of those were stressful, but maybe trying to spread it out over 24 months, making buys every month amid all the market and global turmoil would have been as bad or worse.
IMO the important point is have an Investment Policy Statement you review regularly and follow, avoid impulses and market timing, diversify and minimize costs for a long term winning strategy.