I have already prewritten all my secondaries. It took me a few hours per school, but I write pretty quickly. It gets easier as you go along because you can reuse some things you’ve already written (many schools ask the same or similar questions, but you’ll need to adjust for character/word limitations) My primary PS took me maybe 10 hours to write and polish, for reference.
If you’re asking how quickly you should turn them in after receiving them, within two weeks is the general rule of thumb.