I've only recently gotten to appreciate the importance of accurate timing to good lead playing and I've started using a metronome more often to practice. What I wondered though is - how often and how much time do you spend using it? I find that after about 15 minutes (broken into segments of 2 or 3 minutes @ a time) my timing starts to deteriorate rapidly. I don't know if it's the ticking that gets to me or what...any advice?
Ibanez GRG170DX
Line 6 Spider IV 15
To answer your question, I never once had a practice session that purely focused on timing and used a metronome. It all kinda just came with time naturally while practicing songs and other stuff. Kinda like muting. I don't know very many people that actually dedicate their time to focus on muting, it just comes naturally.

However this is probably one of the reasons I advanced relatively slow so I'm not sure I recommend it. My only personal advice to give you is just make sure it doesn't get boring. If it starts to get boring, you will look at guitar as a chore instead of just having fun. Just try to take a song and learn it so that you are able to play it without any backing track. Eventually you will learn to "feel" the timing and it will be 100% natural.
I always warm up with 10 minutes of pure technical timing excercises

boring, but gotta do it!
Specific practice? I haven't; rhythm comes naturally to me. I always nailed that silly postman's test in Majora's Mask.

However, I have practiced hand synchronicity that way. I set up my metronome on my netbook and sit in the lounge and watch TV, with the tick-tick-tick fed into my ear via earphones. I did that maybe three times, for about two hours each just watching TV and playing anything I felt like to the metronome's beat. When I got comfortable with 16ths at a certain tempo where my hands were in perfect synchronisation, I'd up it by about 5bpm. I didn't progress much in regards to speed, but I actually didn't need to; the speed was there, I was just unsynchronised.

...But now I am. And it was all thanks to the clicks in my local metronome.
Ibanez RG2228 w/ EMG808Xs | Line 6 POD HD500 | Mackie HD1221
I found a few good tips from my teacher helped me out fast with timing.

-Get all the down and ups strokes in the right place.

-Sing the riff/ excercise/ song along to a metronome to get the sound and rhythm into your head, and dont move onto playing it until you can do that in time.

You can also practice just counting the rhythm to a metronome, so if its constant 16th just go 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4, with the 1's on the click.

With timing you really want to just be able to feel it, not think about it.

And I dont know how long I spend on that, I just do it when I need to.
I do spend a little time on it. Probably do an hour a week with a metronome. I take a basic exercise or run through a scale, throw the metronome on at a moderate tempo and then play through different note sub divisions. I use Bounce Metronome software, so i play around with different rhythmic patterns as well.
I dont really practice timing. But i test timing when writing riffs. Like playing a riff on the beat or against the beat and such.