But it's so easy to make a good estimate, takes less than 10 seconds:
Take your instinctive estimate.
Increase units by one (if you think "hours", make it days. If you think "weeks" make it months, etc.)
So if you think it'll take 2-3 days, tell your manager it'll be ready in 4-6 weeks. Don't forget that in management school, they teach these fuckers to under-promise and over-deliver. He understands.