how long would it take to rebuild a push mower?

The engine can be rebuilt for about 50 bucks and should only take a few hours to do if you are skilled at doing it, they are pretty simple engines, you may need to strip the deck and prime and paint it that will take a couple days to do properly and the cost would be the paint and your time.

I would guess an entire rebuild on a mower to be 65 bucks or so total if that.

I used to work in a small engine repair shop and I could rebuild it in 2-3 hours, so a noob could do it, but it will take a little longer, also it depends on engine type, if its a L head Briggs or Tecumseh they are simple, now the overhead valve and OHC engines like Kawasaki and Honda they are a bit tougher but not too much.

Depending on the parts needed and engine type a good estimate would be 50-100 parts only. If it needs boring add another 30-50 etc. There are specialized tools needed for some repairs and assembly etc. Normally takes a 4 hours complete. Sadly in todays world it is a throw away society, and it is actually more economical to buy another one. The manufactures make these things by the hundreds of thousands and this time of year is a good time to buy one, the retailers will discount them to clear inventory and eliminate their storage space. Hope this helps, good luck either way