I've heard of submissions periods closing when they have enough content, but in my experience those tend to be periods with soft deadlines and they mention so in their guidelines. (arsenalofwords.com)
If you send a query letter during the open submissions period, I do try to respond within three or four weeks. (twilighttimesbooks.com)
The very first thing listed under «guidelines» is the fact that we have set submission periods. (nocturnal-lives.com)