I've been curious on the general community opinion on this. How many mistakes is it fair to make before you're genuinely holding the group back and should actually leave.

From experience, even "kick = mistake" groups don't always kick. Kicking someone isn't pleasant and many players will simply leave or disband to avoid doing so, or even suggesting to do so.
I don't pretend to be a good player myself. I've cleared this Savage tier, yet I make mistakes fairly often on much easier content that I shouldn't. In these cases if it happens 2-3 time without improvement I generally try to do the fair thing and drop group for a better player.

I don't apply my own rules to others (I'm very patient in instances), but not everyone feels the same so I try to respect others time. Obviously there's some variance for farm/practice, but how do others feel on this? Is there a point where you go from friendly tolerance to secretly wishing the player would do the decent thing and stop holding the team back? If a player is holding a team back and leaves or is kicked, should they try again in a new group to learn, or move to easier content to brush up first rather than risk wasting another 7 players time?