The flying time is roughly 11/12 hours (Oddly slightly longer on the return journey). April is a great month to go, nothing quite like visiting the temples and summer palace amid the cherry blossom and incense! It will be like spring in the UK, which means you can get showers. I would recommend packing a light weight travel cagoule, but you can buy them there from street vendors who pop up with them if it rains. Enjoy.
P.s. either take an empty suitcase or buy one when your there, the silk markets have un-missable bargains