How long does combat take in the system and how does the spellcasting work. Players have to stay engaged so you can't have them just be "casting a spell" and doing nothing else for any "action window", whether that's a turn or however your game measures time. The maximum is what 3e does with "1 round" (as opposed to full-round action) spells where the player starts the spell on one turn and gains the benefit immediately at the start of the next -- the impression of casting time, but no dead turns.

To do longer cast times you have to give the player actions to do during the casting time. That could simply be delayed effects (you cast the spell, the effect doesn't occur for some time but you can keep acting during that time -- perhaps, if you want to get fancy, with a reduced subset of powers) or maybe they have minions to control or maybe you even give them a separate minigame to complete the spell but that separates them from the rest of the game which is bad.

As far as 5e vs 3.5, yes 5e simplified and lost some articulation as a result.