The old joke estimate, which seems to come out accurate more often than it should is to double the it, then up the units by one (e.g 2 days => 4 weeks)
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Yeah, this is the only answer needed really.
Estimate your work, double it, tell management as a guideline that when you'd like to be finished.
After 2 weeks (or however long your sprint is) estimate again.
Couple this with daily stand ups and estimations are fine.
The old joke estimate, which seems to come out accurate more often than it should is to double the it, then up the units by one (e.g 2 days => 4 weeks)