asics perform the task of mining . if bitocin hash mechanism stayed the same. but it was just a boycot of asics. then there would still logically be someone with an asic that ignores the boycot. so based on there being just 1 asic running

1 asic is about between 110terrahash and near on 300terrahash, where there are about 1.5m asics to get to
current network hash which is about 220 exa


so going from 220E starting point... in 1 fortnight can only drop 75% max difficulty change
so
fortnight 0   220,000,000 T
fortnight 1   55,000,000 T
fortnight 2   13,750,000 T
fortnight 3   3,437,500 T
fortnight 4   859,375 T
fortnight 5   214,844 T
fortnight 6   53,711 T
fortnight 7   13,428 T
fortnight 8   3,357 T
fortnight 9   839 T
fortnight 10   210 T

so ~20 weeks or 5 months to get difficulty down to reasonable level of a single miner.

but that said. unless the code is broke where only one miner can run.. you will find as the difficulty drops, it then makes more opportunity for competitors to jump back in so as said unless the protocol code is broke. others would join back in before it got down to 1 miner

as for CPU mining which is about 20mhash which is like 10m to get to 210thash
fortnight 10   210,000,000 M
fortnight 11   52,452,087 M
fortnight 12   13,113,022 M
fortnight 13   3,278,255 M
fortnight 14   819,564 M
fortnight 15   204,891 M
fortnight 16   51,223 M
fortnight 17   12,806 M
fortnight 18   3,201 M
fortnight 19   800 M
fortnight 20   200 M
fortnight 21   50 M
fortnight 22   13 M

so from starting point down to CPU solo mining. would take 44 weeks or ~10 months