Just because you spend them on summons doesn't mean you will get anything useful. General rule is to expect disappointment. You could get something good, or you could get nothing but trash and end up having been better off using them for something else later.

There really shouldn't be any "suggested" time to spend them on summons. The only "worthy" time is when there is a spotlight unit that you can't live with yourself if you don't at least try for her (or him).

In all honesty, if your goal is to complete the event by summoning something, then don't. You can instead use them to activate the powers/blessings during the event that will make your ENTIRE team stronger should it be necessary. You don't have that much time before the event starts anyway. I'd focus on my current team, and then purchase the buffs if I can't handle the event maps. At least this way is less of a gamble. One semi-strong unit is not likely to make up for an entire team of weaklings.