Difficulty is adjusted every 2016 blocks. The intention is that it will take the entire network 10 minutes (on average) to find a block, so finding 2016 should take 14 days. If solving the next 2016 blocks takes only 7 days, difficulty will (roughly) double.
Whether this is a scam or not, I wouldn't buy any mining device that won't be available for four and a half months. There's no way of telling how high the difficulty will climb by then.
At the current rate, it's raising 30% every 11 days. That's 12 difficulty adjustments until February 24 (if they deliver on time), giving a difficulty of 4.4 billions.
Yes but does diffulty also lower down if noone mines or if it takes more than 10 minutes for a block to be found?
Yes, it does.
if so then when difficulty reaches such extreme numbers I suppose that most of the community will drop out from mining... thus forcing difficulty to low down again then attracting old miners to return difficulty raises again eventually people will drop out.. etc etc wouldnt it be a good idea to have a 2TH/s machine and "come in game" when difficulty raises so high making people to quit and gaining profit by mining while the difficulty level drops (because of less miners in the game) ?? and then take a brake again when people come back and raise the difficulty level?
If it only was that easy.
You'd have to be absolutely brain-dead to mine BTC with a GPU right now. That leaves a couple of brain-dead individuals and a lot of people with ASICs.
Since ASICs can't be used for anything else, you can do exactly three things with them:
1) Sell them.
2) Unplug them and let them rot.
3) Just leave them plugged in.
Option 2) is the only sure way to lose money, so most people will go for either 1) or 3). In either case, the difficulty doesn't drop.
Add to that the vast variety of ASICs that are being sold right now and the fact that every couple of month there's a new device that is thrice as fast and only uses half of the power. Eventually, even a 2 TH/s might be nothing more than a very expensive paper weight. Judging by the pace the difficulty has been rising lately, that moment might come too soon for you.