Short
answer , A LONG TIME. My first one lasted 3 years , what failed was the 16 gal barrel got rusty at the bottom edge and tilted, which caused the riser to crack. If not for that happening that riser would still be in use today. One of the nice features of building a riser this way is the ability to uncob and lift it off of a core if (or when) you go in to modify your core unit.
Of
course if you can afford it the "newest... best.." thing in
RMH's is 1" thick ceramic fiber board or tubing being used for cores (other than the
feed tube) and to build risers. The downside is the cost... I haven't spent the money on ceramic fiber boards yet myself, but the performance reports are so good that they will be used at my home sometime in the future.