It's not time that's the issue, it's the opportunity to boost the self-confidence of the stude and to start developing captaincy.
Yes there's a small risk, but it's significantly less than sending people on their first solo and IMHO the benefit outweighs it. If we are never to trust a stude to be alone in an aircraft with the engine running then the first time they are alone is on that first solo, and the first time they start or refuel by themselves is when they're on their first un-checked solo circuits - that's too big a step.
HFD