If it's been in service for 8 years it's time to swap the anode no matter what (it's 8 years old, after all- I usually swap 'em at 6 years even without the water softener.) It's condition may give you a hint as to how much water softener is hurting (or not- it's old). A DIY anode swap is less than $50, often less than $35. Once the anode is truly toast, broken off with just a stub hanging, tanks can go downhill as fast as Bode Miller. With a water softener system swapping anodes every 4-5 years might be necessary- it just depends. If the old one is still intact when you pull it, put date tag on the new one with both the installed date and an "replace by" date. If it's bare wire, shredded, falling apart, it's probably time to replace the beast before it leaks.
Efficiency typically falls off over time though- in high fuel-cost areas (or with propane versions) it's cost effective to swap in a new one at 15 years even if it's well maintained. The center-flue heat exchangers inevitably corrode a bit on the fire-side, and lime up on the water-side leading to a lower average combustion efficiency. They start out life at ~80% steady-state efficiency, but it'll be 75% or less after more than a decade of service.
The draining schedule is a bit ambitious and unnecessary- annually is usually "good 'nuff".
How much longer it will last is always a WAG, but if the burner burns clean & isn't rusty-crusty and the anodes are swapped on a schedule they can easily go a coupla decades. If it's never maintained 10-12 years would be typical, but even some of those can go 20 years depending on volume of use and the water quality. Others can die within 6 years.