https://badmofo.github.io/ethereum-mining-calculator/Pretty much what every calculator says.
I'm not mining Eth, but I do solomine everything I can and personally, I draw the line at 12 hours.
That is if it takes longer than 12 hours for me to find a block on average then I switch to a pool.
Even with a 12 hour average block finding frequency the variance can be killer. You could go for weeks without finding anything if you're really unlucky. Of course, the opposite can also happen.
Basically the longer your expected block finding frequency is, the more time it it will take (days/weeks/months) for things to even out.
It's up to you what level of variance can you handle.
But, with Etherem one thing to note is the very low block times (in comparison) which means you need a very good connection (very low latency with good routing/physical location) otherwise the blocks you're solved could end up orphans because someone else with a better connection beat you to it. I have a great connection but that alone would tip the scale for me not to bother with solomining. But if you do decide to solomine, good luck!
Thanks for the feedback! Yes, 12 hours, or even 6 hours might be more to my liking. I used to run private pools back in earlier days (Doge and others) where myself and a few like-minded friends would point our miners, so I know all about the good connectivity issues.
This is why I am looking for a good hash-rate benchmark. So at 12 hours, we would need to be looking at 2 GHash/sec which would move me past solo mining and looking more to a private pool with some friends. I would probably host the pool at an online hosting provider to get the good connection you speak of, so this extra cost would need to be accounted for in profitability as well. Still would like to hear from anyone actually solo or private pool mining on hashrates, luck, etc.