Not that much probably.
Both Melee and Caster minions start with 0 Armor.
Melee minion armor increases by 2 and health by 20 every 3 minutes. So for example at minute 12-15, melee minions will have 578 health instead of current 535 health and 8 armor. This is a really minor difference. Of course, if they game will drag on - the difference will be more noticable, as in minute 45-48 melee minions will have 982 health instead of 755 health and 24 armor.
Caster minions armor increases by 0,625 and health by 7,5 every 90 seconds (no, I don't get that either). So for example at minute 13:30-15, caster minions will have 368 health instead of 350 health and 5 armor. In minute 46:30-48 they will have 612 health instead of 515 health and ~19 armor.
For siege minions it is gonna be a bigger problem though, as both their health and armor are rather high.
And anyway, IMO Life Steal should be changed into a in-battle sustain mechanic. So, that it's good if you attack champions and it can sustain you, but it should be a mediocre choice between fights (it's best choice for both right now). Of course, it could be done with simple "Lifesteal have 30% effectivness against minions" and with somewhat buffed numbers.