IME I think it's good when you already have a decent job so you can actually afford to help others sometimes without charges.
Whenever you feel like it actually :)
The same kind of work that is usually only done professionally to make a living, can be perpetually out-of-reach for any but the most well-heeled enthusiasts.
Plus if you are one of the true professionals, then doing a little bit more of the same independently, on top of what you are already doing for a successful employer, can be one of the lowest-hanging most meaningful contributions to the individual who has no other place to turn. Even more so if there was never any alternative within reach to begin with.
This can be one of the most amazing gifts that's given far too seldom.
It can be kind of tough to fix things that conventional techniques have failed to fix, you're never going to hit 100% and I'm the first to admit how much I depend on luck, sometimes in unexpected ways. But that's what I had to work on for myself professionally. There can easily be the surplus momentum to take on a little more of the same or something equally appreciated, for the right recipient.
I was pretty pressed for time so I really couldn't get too involved with things that others could fix better anyway, but then I end up seeing more things that nobody even knows what is wrong :\
That is so critical that the appreciation can be worth more than money, and if you never give something like that a chance, you could miss out on a lot. Individuals almost never came close to affording the rates for lab work that I was charging to multinational clients, and in the long run it didn't make any serious difference if I did a certain amount of work free from time to time. That wasn't my business plan at all, but the plan I did have was working well enough that I could look at year-end growth and returns without having to worry about every single thing I do having to make money in some way or another. There was almost no incremental cost to me to further extend my capabilities in the right situation, and it can mean so much.
Plus I thought it was a good idea to build a company that did not falter if a major project turns out to be very costly and then makes nothing back. Which will sometimes happen so you should be able to justify doing the small stuff with pride.
I didn't usually tell fortunate people that I wasn't going to charge them for the sometimes heroic, last resort efforts, until I had solved their problem and it was squarely in the rear-view mirror.
When people find out anyway, you can be hit with overwhelming needs but one person can only do so much, and then you get a much wider (but more diluted) mix of challenges that you might want to choose from and for who. Increasing the odds of easy success, which can be a very satisfying form of success.
After decades of hammering away because that's what I do, I guess that's a lifetime of pretty good luck at fixing things that nobody else can fix, when I put my mind to it.