Research has shown that there's no definitive window you need to eat after a workout to help your body and muscles replace and repair itself. However, you're going top need protein to rebuild and repair your muscles and you're going to need carbohydrates to replace the energy you've lost after your run.
As we've said earlier if you're running for less than an hour then you can wait until your next main meal. If you're hungry in this window then there are protein and carb bars available that would help in this situation. These are also great immediately after a workout if you've been exercising for an hour or more.
So there you have it, you shouldn't feel hungry while running on a treadmill but if you do then an energy gel is ideal. This is especially the case for longer runs of an hour or more. The gel will have a lot of carbohydrate to keep you going and some sodium to replace your electrolytes you lost during exercise, all this helps you keep going.
There's no strict rule on when you should eat afterwards but if you're hungry immediately then a carb and protein rich drink or food would be best. Make sure it's not too much if this is not a main meal. Otherwise wait for your main meal especially if your workout wasn't too long and also if you're trying to lose weight.