How much work is involved when rebuilding?

The top end part of the rebuild is mostly labor. I usually pop the valves out, run them on a wire wheel to get the carbon off, clean out the exhaust ports and combustion chambers then lap the valves back in. It's a lot of work. G-Force did the original valve job and I've been using the same valves and seats since I bought the bike. Their porting work is amazing. The biggest increase in hp you're ever going to see out of a Ninja 250 comes from good porting and then selecting the right main jet.

I spend way too much time doing rebuilds because I'm totally anal about getting everything clean and making sure all the gasket surfaces are pristine. Saves a lot of time chasing leaks later. I can take a raceable race bike, drop the motor and strip it down to the main bearings in about 2.5 hours. Then I spend about 4 solid working days cleaning and building. In between are several unexpected parts orders. Now that I'm rebuilding for the 5th or 6th time, there are less unexpected parts to be ordered, but always some sort of gotcha.

Note: Unless you're racing, you might consider swapping out a low-miles used engine from a "donor bike" on eBay or from one of the used parts sources. Of course, if you always wanted to rebuild something, this is your chance. Get a service manual, though. This does not fit the definition of "frequently".