Hi Darren,
My propoagating bench is full now with germinating beetroot, onions, spinach, lettuce, peas for shoots, brassicas etc. I leave most trays on until seedlings are clearly established but it depends how many new sowings are about to happen. You can leave trays on until almost ready to plant, just lift occasionally to stop roots going into the sand, and with peas this needs doing every three days or so.
In essence, I like to keep it full and use all the heat, with no particular time that is best for plants to come off, once they are germinated.
good sowing, Charles