It's a good question, and the best answer I feel anyone could give is to say to you....you try! This is because it may well be that in any particular location, and due to the type of climatictic conditions you experience in that particular location, and whether you are in a climate controlled shop, with the type of blend you are using, that this will vary a little....but really, you will probably find somewhere up to half an hour is as far as you would realistically wish to leave it....depending on how pedantic...or how pickie....or particular....or astute...or fussy....or whatever...the individual may be....particularly as many cafe owners are quite happy to leave the thing full, all night, and start again the next morning with last nights grinds, without a care in the world or without apparently noticing or caring that there might be any difference in the coffee thus brewed, between it and coffee brewed with fresh grinds. That's life! Regardz, FC.