A 2.5 million immediate annuity pays $10,425 per month for a 50 yo male.
I don't see how this would not be enough, then again I am a "frugal" type.
I am not recommending annuities, just using it as an example.
At 3% inflation:
In 10 years (age 60), that $10,425 becomes $7,780
At 20 years (age 70), that $10,425 becomes $5,760
At 30 years (age 80), that $10,425 becomes $4,290
At 40 years (age 90), that $10,425 becomes $3,198
The immediate annuity's payment can give you a false sense of security. Unfortunately owning an annuity for long time means that inflation will likely have a significant diminishing effect on the real value of that payment.
I think that a methodology that uses an SWR for a 40 year period (probably around 3% as others in this thread have suggested) gives you a better idea of what you can actually spend. That would be around $6,250 a month.
This SWR is designed to keep your payments increasing with inflation - so there will be no decline in the real value of your payments (your nominal payments will be increasing, so they'll end up at $20,375 in 40 years, assuming 3% inflation).