7am should see you pretty early in the queue for most things. I've dayseated at the Donmar many times, and I think my most surprising result was getting through the door of
Othello after turning up shortly before 8.30am. Sure, I only got standing room, but considering what a hot ticket it was and how early I now start queueing, I'm absolutely flabbergasted I got in. XD
I think it's safe enough to say
Lear will be a hot ticket, so I shall not be letting on what time I plan on arriving (let's just say it'll involve a night bus and leave it at that), but it'll be winter, so that should frighten off less hardy theatre-goers. 7am sounds pretty reasonable, especially if you're talking a midweek performance earlier in the run. You might not be first in the queue, but you should get a seat.
My top tip for dayseating is: decide what time you'd like to start queueing (let's go with 7am for example), then turn up ten minutes earlier. People tend to think in round numbers; mark my words, if you roll up at 6.50am, you'll have a good place in the queue, and you'll feel VERY smug when tons of people all roll up at the same time a mere ten minutes later. XD
If you don't mind standing up, the standing spaces are actually better than the seats. But there's nothing wrong with the seats, apart from being side-on; I've sat in B41 I don't even know how many times now and the only problems I've ever had have been actors standing right on the corner of the stage, and they hardly ever do that. :)