I recently answered a question with a proposition that the asker should improve his resistant-to-change boss's legacy system by coding the alternative in his spare time and then presenting it as an alternative approach to his peers.
It got me thinking about all the unpaid development work I have done in my working life. Although I know it is our character to work late, in darkened rooms, eating pizza and slouching in front of a couple of monitors when do you shutdown and go home?
I have the luxury (burden?) of working from home so I end up doing quite a bit more work than I would were I in the office. With the laptop already set up and connected to the VPN, it's entirely too difficult for me to resist the temptation to "just pound this out". I'd say I probably average a few (less than 10) hours a week of "unpaid" work. BUT, I will only work "unpaid" if what I'm doing is at my own direction or to help fill a gap in my knowledge that is relevant to the current project.
The little "oh, I should try it this way" moments would qualify for "unpaid" time as would the "now, how the fsck do I do that?" problems.
As one of the comments on that question said, why not put that time into a project of your own? The real answer is an indicated of how much you have "signed up" to the project. I give two examples
1) A brilliant project where we did 6 impossible things before breakfast. The PM managed to get us all (and I mean everyone) to work an all nighter, then do a long day, then home and restart at 4.00AM in time for a 12:00 demo. The things is we loved it, and reaped the rewards later.
2) A desperate mess going nowhere, demoralised team, a clear mandate to fail from senior management. At that stage what can you do, long lunches, google, and CV work. No work on out of hours time at all.
It all depends on the project.