At what point does solo mining become worth it, when you can generate at least 1 block per day? For Bitcoin you would have to have a ridiculous mine, but for something like Litecoin that's achievable by spending less than $40k on gear.
I would suggest that unless you are doing it for gambling reasons (like all those mining on my pool) being able to mine 5 blocks on average per diff change - this would circumvent the potential losses of diff rising and you not finding a block in the previous diff. At current diff of 678G that would be over 10PH now...