Proposal
Characters begin with a single year of craft days (365 days) which are expended when crafting items (magical or not) outside of an adventure at the normal rate. One craft day per 1000gp in the items base price, with a minimum of at least 1 craft day; potions always only take one craft day. See the creating magic items rules
here.
Characters gain one craft day per real life day not spent adventuring. Items crafted while on an adventure do not subtract from a characters craft day total, but crafting in an adventure does take up time in the adventure timeline. Please check with your DM and the adventure's judge when crafting an item during an adventure.