TomD ... First of all, happy early birthday. Secondly, the short answer to your barrel replacement question is:
1. If/when it becomes physically damaged in some way (poor maintenance, corrosive ammo, dropped, improper ammo, etc. - not round count related)
2. When it wears (because of round count) to the point where loss of accuracy becomes noticeable.
In relation to round count only, I have heard of many barrels going a 100,000 rounds or better without issue. From a personal perspective my highest original barrel round count pistols in my collection are:
1. MKIII512 at nearly 70,000 rounds
2. GSG 1911-22 at 31,000 rounds
3. P95 at nearly 13,000 rounds
Bottom line, if you properly maintain your SR9, the barrel will likely outlive you
