There are two time considerations here. One is how long it takes to put the parts on the bike, which isn’t a lot if he’s worked on bikes before and has access to the right tools.
The second, more important factor is the time it takes to get all the parts if you’re looking for deals and buying one at a time. If you want to keep your costs low, you have constantly check for good deals and be patient until they come up. That can certainly take months.
This is also a bad time to be buying a bike, as the warming weather creates a lot of buyers.