No, that is not normal at all. For my 5:25 the first power tube set lasted 11 months and that was with 2 hours a day playing at home, a jam and a rehersal each week and at least 6 gigs. Total use would have been around 800 hours. The amp was still working fine too, but when playing quietly at home I started hearing very faint pops every so often. The pops started to get a little louder over time so I just decided to get new power tubes. The second set are still going strong 7 months on.
I don't think there is any way to tell if you have had a bad run with tubes or if something else is going wrong without getting out the test gear. Is there anything physical going on , does the amp get bounced around in the back of a truck or something ?
Still it does sound like it should be looked at by a pro. Hope you get it sorted.