That’s about right for newbie gains only (your first 20 lbs or so of muscle). Once you become even an intermediate lifter, it’s incredibly slow to add further muscle mass without the use of drugs.If I'm not mistaken, you are able to grow 1 Kg of muscle ever 2 weeks if you do every thing right (sleep, work out, food) which means you can grow about 2 Kg of muscle every month under optimal circumstances. So if you did gain 10 Kg in a month, I'm afraid 8 of those are fat in the best scenario.