It's different for every person.

We can do some math to try to predict it tho.

Suppose we have $100 and we're playing $5nl at 20 buyins, we want to move up to $10nl at 25 buyins. So we need $150 more. If our edge is 5ptbb/100, we make 5 big bets (.50) per 100 hands. If we play 500 hand sessions, that's 2.50 per session, so it will take us 60 sessions to get to 10nl.

If you just moved up you might want to just subtract a little from the edge you had at the previous limit rather than take a small sample set at the new limit (which might be a loss anyway).