They can be completed at any time, just like the main story quests. Mobs will be scaled to your level.

I would suggest just completing the map quests, and do the guild ones as you come across them, which you should do if you follow the map quests in order.

They are spaced out in a way that if you play quite regularly, you shouldnt forget whats going on in them, and it's a nice bit of variation.

But yeah it's up to you. You could wait until a higher level so the weapon and armour rewards will be more useful and not replaced almost instantly.