Glad i could inspire you, well i haven't run the shirts i just sprayed through the wash yet so i can't tell you how those specifically will hold up yet. But i've made shirts in the past and never have had a problem with them fading to much, atleast never to the point of just being a faded mess. As long as you heat set the paint into the fabric it should have some pretty solid staying power. To heat set the paint, ideally you'd use a heat press which will pretty much guarantee some staying power, but most people don't have access to a heat press so a good alternative is to run it through a dryer before wearing it or washing it, it's not quite as effective as using a heat press but it does the job well enough.
Now depending on what you use to print with i've had different experiences with getting them to last. With screen printing i know there's a fixative type substance you can mix in with the paint that eliminates the need to heat set it, i've never tried it personally but i have a friend who has a moderately successful clothing company and that's what she uses, so i assume that works well. But just trying to heat set a screen printed shirt in the dryer hasn't always worked for me, i've bought/made screen-printed shirts and had them fade to shit in a month despite running them through a dryer.
With spray paint however, i still have shirts i made a year ago that i never heat set, and they have only faded a little, but it only just has a faded aged look, not faded to the point it's impossible to see, the colors just aren't as bold as brand new. So just from experience spray paint has some pretty good staying power on shirts, i've heat set both of the last two shirts, one with a dryer and one with a heat press, so i can get back to you on if they fade quickly or anything.
Hope you're not to dizzy after reading all the mess :) I tend to get carried away, glad i could inspire you! Hope this helps.