。
It's a 15-minute walk.
Literal
Walk-and 15 minutes is (polite).
The minimal form — 歩いて + duration + です. This compact formula is a direct answer to どれくらいかかりますか and doesn't use かかる at all; the time is simply asserted as the predicate. Extremely common in real-estate listings ('駅から歩いて5分' = '5-minute walk from the station').