Yup , hexs can be used as often as you like. The only real restrictions are that some cant be used more then once per enemy per day.
Edit - a good example of this kind of hex is the slumber hex.
A cute houserule for GM's that think the hexes are "too powerful" (which they may be if a large percentile of the opponents are humanoids) is to allow only a certain number of uses of hexes per day. For example the limit could be: 4+Int mod+2/witch lvl beyond first uses per day. This is similar to the number of bard song rounds. These are a lot of uses and a witch is unlikely to run out but it will give a player pause and not use a hex on absolutely every goblin they meet.
Without house rules, there is no limit on number of hexes per day.