どのぐらい is used to ask about an approximate number and quantity in general. Here, it asks about an approximate duration of time. どのぐらい can replace such words as なんかげつぐらい(about how many months), なんしゅうかんぐらい(about how many weeks), and なんじかんぐらい (about how many hours).
A:がっこうの ふゆやすみは なんしゅうかんぐらいですか。
How many weeks is winter vacation at your school?
B:がっこうの ふゆやすみは どのぐらいですか。
About how long is winter vacation at your school?