Depends on certain event. I play, for example, roulette - once per 3-4 days and coin dozer - once per 2-3 days. Its up to you how many times to play, but charges accumulate within the whole event week.
There are cumulative chances there. You may play most events once a week if you wish.
Probably items were put on some hero - who knows. It's normal when the game warns you or forbids to dismantle such hero in this case. But not this game...
This game is a scam. Gacha ratios is a fraud. Got 1 hero from 12 summons, it's a 0.8% chance, but they say it's 3% ratio. Scam.
Worst developers/publishers I've met so far. They turned events on Saturday and turned them off in 2 days, on Tuesday.