How many days to reach ROI?
anyone knows?
Mining bitcoin: never, as common sense would suggest.
Common sense does help occasionally. However, in this case maths is more helpful. This is the simplified formula for SHA256 Bitcoin mining:
ROI_limit = BTC_per_block * 1 THash * seconds_per_day / (Difficulty * 2^32)
At the current Difficulty of 144116447847 this is
25 * 1000000000000 * 86400 / (144116447847 * 4294967296)
or
0.0035 BTC/TH/sec/day (approximately)
In other words:
If you want to
buy hashpower then you shouldn't pay more than 0.0035, otherwise you'll lose money in the long run.
If you can
offer hashpower then you should ask for 0.0035 or more in order to get an advantage over mining at the average mining pool.
(Don't forget to take the appropriate fees into account.)
After the next difficulty change (around Feb 19th in the evening, UTC) this ROI_limit will most likely be 0.0031 BTC/TH/sec/day.