I find it initially takes effect +40 mins, peaks around +70. First time I did it I was as high as a kite for a good 2 hrs, maybe a little more because I snorted a booster dose about 1.5 hours into it. Subsequent doses have peaked for about an hour, then leveled off for another one then tapered off. Re-dosing does keep you in the 'high' zone of the experience but, as has been mentioned above, causes more speed-like effects and trouble sleeping. To be honest I haven't had any without a re-dose so I can't comment on a single doses duration.
I've had about 400mg today, 200mg orally then 200mg bumped in smallish lines every 15 minutes or so. Dropped at 12, re-dosed around 2, was high til about 4.30-5. Chilling and smoking weed now and going for a few beers with the boys.
As for the paranoia I was discussing earlier in this thread, I'm putting it down to plain old ganja, not the mephedrone. Maybe the combination of the two didn't help, but I've been smoking too much lately in general and its beginning to manifest itself in the ole noggin.
Also, as someone mentined earlier in here each subsequent dose seems to be less effective. I reckon the magic is fleeting with this one.