Well, I've never had to go more than 4 or 5 passes even on the worst content. Setting the amount of noise reduction too high results in artifacts on the programme material which, to most ears, are more disturbing than a bit of hiss.
I'm afraid it's important to get a new noise sample after each pass. You're changing the character of the noise each time you run noise reduction and the new noise sample allows the system to work properly. Similarly, changing the FFT size (upwards) each time will improve the performance of the noise reduction each time, especially on higher frequencies.
I've never had the problem of not being able to find at least a little true silence on a recording, even ones done in live theatres. Places to look are just before the orchestra starts playing (the audience is usually at its quietest just as the conductor raises his arms. Alternatively, there's sometimes a gap after the last note and before applause.
However, if there's truly no silence you can use, perhaps you could run the NR with exactly the same settings as you use on programme material on the blank tape each time, then grab the new samples as needed. The easiest way might be to simply edit some blank tape hiss on the beginniing of your recording while you're processing things, then cut it off when you're finished.