I think once a character's saving throws have lowered enough to give them a 50-50 chance to save against most of its eye weapons and usually after LvL 10. It also depends on the Beholder, is it a fully healthy one, does it have all of its eye weapons available, where is it attacking from, does it have allies with it. After all it only gets ONE attack per round while a party gets multiple attacks and it only has 75 hit points max. Depending on the size of the party and initiative the fight could be over even before the Beholder gets an attack off. It’s not as fearsome of a monster as we are led to believe.