I find 2 things that help learn when to sharpen. 1st is to have a second gouge handy. When you pick it up and use it you can feel the difference. Then its kind of fun to use the other gouge just to learn the feel. If you feel like your having to work to turn then it probably needs to be sharpened. The other method I use is to try and take a really shallow cut, maybe .010" or so. If the gouge comes out of the wood or simply won't cut that fine of a cut then it isn't as sharp as it could be. Now I don't sharpen 25 times per bowl. I take pretty large cuts when roughing so I can usually get it close to the final size on one sharpening of say the inside. Then I will sharpen and make some cuts down to final size looking for any tearout or how clean it's cutting. Then I may sharpen again to get the final cuts and need be one more time. Same for the outside. Kind of depends on the wood. I turned an Ash bowl the other day that was really hard. Took quite a few sharpenings to get through that one.