Choosing whether to stay for a possible promotion that has no firm date

I've been told I'm ‘next in line’ for a promotion, but no one can say when the role will open. I like the work, yet I've passed on external interviews partly because of that possibility. I'm trying to separate encouragement from a concrete plan. What questions would you ask your manager—budget, required milestones, decision date—before deciding whether to keep waiting or explore other opportunities?