I have 3 1982 GTE Sylvanias in my Dazor 318A3 on my desk. On average, its gets 2 cycles per day totaling around 8 hours per day. When I turned them on I hold down the rest button for 4 seconds and the tubes almost always start without issues.

If the tubes are rated for 7500 hours at 3hr/cycle, how long should they last?

I've seen many desklamps that still run very old lamps so I'm hoping mine will last a long time!

I have two F15T8 lamps that are Hungarian GE, made in about 2009 or 2010 and have had roughly a couple of hours every morning plus roughly dusk to midnight service since early 2011. Both are on choke ballasts with FS-2 starters. There is noticeable end bands, but they are still going strong. The package also indicated about 7500 hours at 3h/cycle. You should be good for lots of years.

That's good, I'm pretty sure manual start can be softer on the cathodes too compared to starters!