Depends on the complexity of the game and whether you want to play against the computer or against a mate:
I made a simple game (e.g. tic-tac-toe or console snake) takes about a day for a mid-level front ender, and then I have made something more complex games using threads that took me about 3 days (e.g. a simple bat-and-ball against the machine). And I started a TETRIS game about a year ago and am still busy with it...
So you are working in the browser? You can do some pretty cool stuff within bounds... I have seen some pretty amazing animated games with amazing illustrated backgrounds. If you are creating "platformers", you can probably carry on forever; releasing a new game with new stages as you complete them... Problem is that people so easily get bored. But then I still play TETRIS every now and again, and some people might refer to that as an "old game"...