I was under the impression that くらい is an adverbial particle (副助詞), but here it's acting as a noun (名詞)? Can it also function as a noun, or is there an implicit noun after くらい and before だ that's being colloquially dropped (maybe something like 時間)?
Here's my rough translation attempt:
It's an approximation (くらいだ) of until (まで) I return home from school and sleep (学校から帰ってきて寝る).
This is pretty awkward :) I assume the sentence actually means:
It's the rough timespan of from when I return from school until I go to sleep.
It's roughly from when I return home from school until I go to sleep.
But if that's the case, why isn't there another から after 学校から帰ってきて? I'm not sure if this is grammatical, but I'm tempted to parse the sentence as
- (学校から帰ってきて)から: From (から) after I return home from (から) school
- (寝るまで)まで: until (まで) I sleep
- くらいだ: it's the approximation