How long in advance should I apply for an L visa? Duration of staying?

Hello,
I'm Vietnamese but studying in the UK. My friend invited me to visit his family in Dali, Yunnan this summer. I have booked the air tickets for arrival on August 2nd and return on September 12th. Also I want to apply for an L type here in the UK (it would be next Thursday 28th June) before getting back to my country. This is about two months in advance to my intended date of entry, which is August 2nd. Is that ok at this time or too early to do so because I'm afraid that the entry permit may expire before my departure or just a few days after I'm in there? And with my planned trip's duration as above how long could my entry permit be? Thanks.