No one really knows, since they tend to live "forever" (compared to many other aquarium species that is - not meant literally of course).
There are a few posts now and again regarding fish over 20, and your's isn't far of 30, so it's quite high on the list.
Sufficient to say that if it's still spawning, it's probably in good steed for another several years. That is of course unless you believe the hypothesis that they get jinxed when you post about them on the web - I don't...
I have a S. eupterus and a S. angelicus that are both around 20 years old, and they still look strong and vigorous. I'm not suprprised to read of a Syno that's pushing 30, but I am surprised to learn from your post that they are still spawning at that age!
yes i can confirm synos if well treated and good water conditions can live 25-30 years and i have heard many stories of these catfish living 25 years plus that seems to be a common feature in the aquarium and a credit to all the keepers who must be doing something right for them to live this long ,
FWIW, the male finally passed in 2019. I still have six of his “babies” left, three of which were born between 1984 and 1986. So even the babies are 38-40 years old now.
My present group of 3 male sibling S. shoutedeni are around 15yrs old (they were about 3-5yrs old when I bought them around 2014.
Absolutely Jools, Cheers to the wise old Syno's!