In qtVlm you go grib->download from VLM, it shows the last 4 (you want to take only the last one), there is the time/date there (GMT), so you know at which time is the next one.
Care there is a temporary grib that appears more or less 1/2 hour before the complete one, containing only a few hours (12 I think). Fanatics are taking this grib to try to guess what has changed. This temporary grib is applied to VLM until the next "5-days" one is availaible (which will have the same 12 first hours). After around 1/2h more there is a grib with 8 days (grib->download), then 15mn later approx you can d/l a grib with saildocs (16 days). Of course 16 days forecast
are a bit hmmm to be taken with care
As is everything over 2 or 3 days.