The main thing is that once it's really dry you don't want it getting wet. We had a sharp shower here last night - I heard it, and the cement was wet this morning - and our friends uphill had a downpour just as they started baling their first hay for 7 years.
When the top's dried, go pick up a pile of it and see what you think. If there's no green left, it's dry. If there's green, twist it hard - does it break? If so, bale it / get it in. If you can squeeze moisture out, it could do with a bit more drying - can you shake it out a bit? (But don't be silly in this heat.)