I prefer to visit DLP during the summer when the park opening hours are at their longest, even if it means putting up with high attendance. This year the summer opening hours (9am - 11pm, and 9-9 for the new park) run from 6 July to 1 September. I've never been able to visit in the early part of July, but have visited in about every week between the last week of July and the first week of September. I've not noticed much of a difference between any of these times. If you are thinking about July, make sure you avoid Bastille Day (is this July 10?).
The best tip for visiting DLP during the summer is to get to the park early. If you are normally like a lie-in on holiday and get to the parks around 10.30ish, and perhaps you don't stay until closing time, then you had better be prepared for long queues for the main attractions (anything up to an hour or more). We always get to the park at about 8.30 or 8.45, have a relaxing stroll up Main Street and then dash about like mad things from 9am until about 11am. This gets us onto the main attractions without much or a queue, then gives us time for an early lunch (once again avoiding the queues) and a great opportunity to relax and take in all the DLP atmosphere. We normally head back to the hotel for a swim or a sleep, before returning to the park for the evening. The last hour or so is also a great time to hit the major attractions as the queues begin to die down. I have a very low queue-threshold, anything more than about 15 minutes makes me scratchy and irritable, and with the right strategy we have never had a problem at DLP. I don't think it is any worse for queues than WDW in July.
Regards
Rob