It will take only one difficulty adjustment to return to 10 minute blocks. In fact it's kind of interesting how satoshi designed the thing would actually make this sort of attack (I call bip 110 an attack), would be difficult if not impossible to pull off. He clearly was on another level.
Because 110ers can claim it's a soft fork and claim every 110 block is valid on legacy chain but them trying to tighten consensus is easier said than done. The need to wait approx 4 years to lower the difficulty or they need to hardfork to a pow change or an artificially lower difficulty. That is good engineering.
