Fair enough. Certainly we can all think of shows that were running on fumes near the end, although, personally, I'm not convinced that comedic shows have shorter expiration dates than more dramatic ones.
In general, trying to figure out when to pull the plug is easier said than done. Shows tend to overstay their welcome (hello, X-Files) or end too soon (Firefly, we hardly knew ye), but hitting it just right? That's a rare accomplishment. (I'm having trouble thinking of a good example right now.)
To me, Warehouse 13
still seems to have plenty of juice left, but, of course, that's a judgment call.