I would say a week give or take depends if you are going to do anything with the brew like dryhopping or finings etc.
If you are not doing anything you should see the beer starting to clear from the top downwards if you have a Coopers type fermenter it will appear darker at the top as the yeast falls down to the bottom + the krausen will disappear and you can see clear beer through the lid or Clingfilm.
The longest I have left a beer in primary is 8 weeks for a stout and it turned out fine.