My first Mac was a Powerbook G3 Lombard I got back in 2009, like many of you, because I wanted to learn about Macs and that was what I could afford. At the time, I was a freshman in high school and snagged it for $100 on eBay. I added a USB WiFi dongle and used it as I would have any other computer for a couple of years. I didn't feel like it felt at all out of date, and I used it to do all the same things I did on my Windows Vista laptop I had at the same time. In fact, I remember being really surprised at how snappy the 400 MHz (IIRC) CPU felt, and attributed it to the architecture difference, especially compared to sub-GHz Intel boxes I'd used.
Then, about 2 years later, someone my Mom worked with lucked into a 1GHz iBook G4 that she brought home to have me service. That thing felt lightyears ahead of my Powerbook, and she saw how excited I was to play around with it. I did a clean install of Tiger, and sent it back to its owner. But Mom surprised me by buying it from the fellow and giving it to me for my birthday a couple of months later. I used it, like with the G3, as I would have any other computer, for a couple of years. I have a fair amount of patience with older tech, so I never felt limited in any way. It did everything I asked of it, including the (then) "modern" web. When I went off to college, it stayed behind and I committed to using my Windows laptop which by that point had been upgraded and was noticeably more "capable", at least in terms of speed.
Shortly after I graduated in 2016, I renewed my interest in the iBook, but I found that by that point, the web had made using it "normally" a bit more challenging. After I got married and set up my home office that summer, it began a life mostly serving as a Spotify jukebox on my desk while I worked. I tried to bring it up to what I would consider "Daily-Driver" capable performance by maxing out the RAM at 1.5GB, but still found that the 1Ghz CPU was struggling to provide what I would deem as sufficient performance for daily "normal" use. I considered upgrading to an SSD, but after spending time lurking on this forum, I decided my money could be better spent tracking down one of the last-gen DLSD Powerbooks. So, after awhile, I snagged one, maxed out the RAM, slapped a mSATA SSD in a cheap PATA enclosure, and was back to feeling capable.
That is still the machine (machine-type at least, as many of you know my first DLSD died) I'm using now as a DD, typing this post on it currently actually. Though there are a few things I use regularly that don't necessarily work as "normal" (thinking about YouTube, Gmail chat as the biggies), but workarounds (thinking of PPCMediaCenter and iChat for those cases) keep me able to use it for virtually anything I want, and my work computer (2013 MBP) and personal Windows desktop fill in the occasional gaps.
I say all that to say this. The Lombard was still feeling decently spry until it was about 14-15 years old. The iBook was about the same, if not a little less. The 15 year cutoff for my A1139 would then be 2020. However, I don't expect the same time period to apply to it. I am always amazed with the community around these PowerPC Macs, especially in this forum. With the DLSD being the last, best PPC portable, I would predict that the awesome community of folks keeping software alive for PPC in general, will still be keeping it useful in two years. By that time, I expect that the large majority of G4's will be what I consider outside the realm of possibility for daily use, but I'm hoping the "latest"
PPC machines will gain a couple extra years of longevity, since at this point it is already love of the platform, and an awesome community of tinkerers keeping them relevant.