Maybe worth a read:
Of course, no one is going to put the "1 million years" to a test, but with a decent quality SD card, unless you are writing a high volume to it constantly, there's no reason you can't run one in a Pi 24/7 for 2-5 years. I've gotten into the bottom end of that range before I switched one out because it had a problem
e2fsck couldn't solve -- I still think the card may well be fine if I reformatted it from scratch, but I do not see the point in bothering since it only cost $5-10 to replace.
The key point in getting an SD card that will last longer is more size.
As in, more than you need. If the total volume of information on the card is going to be ~6-8 GB, use a 16 GB card. If you are worried the user will then just fill it up, format it so there is only an 8 GB partition. If you are using a new, decent quality, name brand SD card, it will have some degree of wear leveling as described in the first link above.
Of further interest about that:
https://electronics.stackexchange.com/q/27619/52138 (I would read at least the top two answers there, which provide a range of information...the second one is probably good for actual hints about brands, e.g., Sandisk seems to come out as a wise choice for a reason.)
This means despite the fact that you've only allocated 8 GB of space, the entire volume of the card will be in use; things that are frequently changed will be moved around, and there will be more places to move them to, meaning the same places get re-used less, meaning the card will last longer.
So, literally, one might expect a 16 GB card containing the same volume as an 8 GB card to last twice as long.
However, if you are someone who is selling other people a product and you wish to be responsible, create something of quality, etc., you should also have a simple system allowing your users to back up to a USB stick or something, and then restore to a new SD card from that. If you are not that kind of person then who cares how long the card lasts, hopefully you can make your money now and not have to care about where things are at in 6, 18, 36 months, etc.