There's no required number of hits needed before Aegon starts getting persistent charges. As long as you quit out while he has a combo it will add the number of persistent charges based on his sig level.
And my advice is don't get greedy. If you lose your combo shield (if running proxima synergy), don't be afraid to exit out and keep those charges. I lost several 100+ combos ramping up because I thought that I could keep going without the shield.