Comment by Zak

Knives should be sharpened when they no longer cut as effectively as the user would like. Several weeks of daily use with no maintenance is probably longer than I would go, but I do not know your knife.

It's not necessary to use a whetstone that often though. My preference is a leather strop with a fine (under 1 micron; I use 0.25) diamond paste on it, and it usually only takes a few strokes (around five) per side to restore my gyuto to performance I'm happy with. The duller I let it get, the more time it takes to restore.