You can start anytime you feel like taking a break from your usual routines, you can start unlocking other classes now by talking to the guild receptionist at each city. Also, if you want to have an early advantage in crafting it won't hurt to level most if not all either of the crafters to level 15 as you can access the first cross class skill of that crafter class for the benefit of another.

When you have access to Guildleves already, you can start gaining exp and money quicker by completing them. You can spend some guildleve allowance on battlecraft for your main combat class if you want to level up slightly faster.