Copypasta from
https://bitcointalksearch.org/topic/m.1884910:
I decided to put together a guide for people who want to see whether solo mining makes sense, based on their hashrate and difficulty. Here's some quick math, done with 1MH/s of (scrypt) mining power.
Difficulty 3.5: 4 hours 10 minutes for a block
Difficulty 5: 6 hours
Difficulty 7.5: 9 hours
Difficulty 10: 12 hours
Difficulty 15: 18 hours
Difficulty 20: 24 hours
To adapt this information to your rig, simple math comes in handy. For example, if you rig is 1.5MH/s, just divide each above time by 1.5 (6 hours divided by 1.5 is 4).
If you rig is 0.5MH/s, divide each by 0.5. Difficulty scales linearly with hash rate of network, and a difficulty of 20 is 2x harder than one of 10. Given this, you can easily calculate higher difficulty. A difficulty of 40 would take 48 hours, and one of 80 would take 92 hours at 1MH/s.
Current BitBar network difficulty is around 1.7