That is a hard question with a simple answer.

You sharpen when the tool is dull. Okay, when is a tool dull?

When the tool is no longer a pleasure to carve with; or you see white streaks in the tools path, or the cut surface is cloudy or hazy and stropping does not restore the edge.

[Captain_Bandaid]

A dull tool shows up fast when cutting across the grain of soft wood. A sharp tool leaves a crisp edge, a dull tool leaves ragged, fuzzy edges. I keep a scrap block of wood around to check my edges after stoning and stropping. Those little white lines in the chip tells me to go back to the stone.

[Kenny_S]

Can't strop enough unless you aren't getting any carving done!

[Colin_Partridge]

I very seldom sharpen my tools unless I get a nick in the blade or cutting edge but I do strop my tools after every carving session sometimes during if I feel that the tool is starting to get a little rough to push thru I will strop to take off the residue that sometimes builds up on the blade I dont think you can over strop but I could be wrong.
Colin

I strop mostly, but occassionally I give one of my most used tools a few licks on a stone and then strop it. It's amazing how one can get used to the feel of a tool and not know the difference between sharp enough and really sharp.