Re: knowing what's right - Just try stuff and see what clicks. I'd never tanked anything before XIV, so I assumed that I'd hate PLD, but I ended up really enjoying the ability to get punched in the face repeatedly and not die (and also to control the pulling pace). So that's my secondary.
MNK just fits with my usual playstyle - I've always preferred fast melee dps (Combat rogue in WoW, Marauder in SWTOR), so playing roughly the same role in XIV felt like a natural progression.
Re: when you get a feel for your class - IMO that process starts early on, but as other people have said, you should probably have a good feel for it by 40-45ish in most cases (BRD changes significantly post-level 52, and there are others that play pretty differently after 50).
Tl;dr - Try what you're interested in and above all, have fun with it.
MNK just fits with my usual playstyle - I've always preferred fast melee dps (Combat rogue in WoW, Marauder in SWTOR), so playing roughly the same role in XIV felt like a natural progression.
Re: when you get a feel for your class - IMO that process starts early on, but as other people have said, you should probably have a good feel for it by 40-45ish in most cases (BRD changes significantly post-level 52, and there are others that play pretty differently after 50).
Tl;dr - Try what you're interested in and above all, have fun with it.
