Originally Posted by Yooper
When fermentation slows down, at about day 5 or so.
Day 5 is fine in most cases, but, to the OP, as you get familiar with different yeasts, OG levels, duration of lag phase, etc. you'll have a better idea of when to raise the temp. I do a D rest with virtually every batch, as it also encourages the yeast to finish attenuating and clean up. I find with WLP002 a D rest as the krausen drops works, but with WLP001 the krausen likes to stick around longer so usually take a reading before allowing temp to rise.