No specific time frame given so far. But from developer feedback in Beta (see below), you don’t have to summon all available heroes for a new selection of heroes. There will be a specific date when the Fated Summon Portal refreshes, no matter how many heroes you’ve already summoned.
No, it didn’t in Beta. So when you are at 95 and do a 10x, it will still show 100 after the 10x. When you select a hero, it started over at 0/100.
Developer feedback from beta:
Also, all information would have been available here already: 🧪 Early Information on Fated Summon [Part of The Beta Beat V52]