It's nearly impossible to plan. Sometimes a very difficult thing is done in minutes and then you want to add a simple button to the gui and it can take days in the worst case.

One time I had to add some kind of grayscale effect to a gui. Well, it was not supported by the gui system. So of course the boss told me to put it in there any way. It ended up taking days.

In the end it wasn't even shipped! At least if it was open source I could have shared the work. So ya, it can be super frustrating. Especially if crunching is expected because your boss thinks its your responsibility.