How long can you leave a crossbow cocked will depend on what you have for a bow & what your doing with it.

Lets talk about in hunting situations. With a quality crossbow, you should not have any problem cocking it in the morning (sunrise) and leaving it until the end of hunting for that day. Remembering that a loaded crossbows should always be treated just like you would treat a loaded gun, always pointed in a safe dirrection. There are still some cheap crossbows that I would not trust to leave cocked for 5 minutes, but most bows today are of the quality kind.

Now if your going to want to store it cocked (not loaded, no arrow), I would not recomend you do this, but make sure that you do not store it in a hot place like a trunk of a car in the hot sun. I was taught long ago that mechanical devices are just that, mechanical and human made and could fail. Always treat them like they could (chances are that they will never fail) and you will always be on the safe side and no terrible accidents can happen.

I know you mentioned in an earlier post that you had a horton (a compound), you need to be careful of dryfires as compounds are not forgiving to the dryfire. BUt you should be safe to use this bow from sunup to sundown (cocked all day) and then let it down at the end of the day. This is the safest for all, you and the bow.

Good luck.