Can somebody tell me by when, difficulty must be adjusted, for the next difficulty change to be >10,000,000?
Or what formula i'd need to use?
No formulas exist -- it's best guess only (there are estimators around, e.g. http://bitclockers.com/miningcalculator). Difficulty is adjusted every 2016 blocks based on an moving average target of 6 blocks per hour. If blocks are generating faster, the difficulty increases; if slower, difficulty decreases. Of course, with all the ASICs coming online, difficulty will probably never go down. The next 2016th block will be Block # 233,855 coming up in a couple of days.