Polyamide is among the good guys against creep. If used far from its yield strength, it should live long. Carlowitz' Kunststofftabellen (excellent but 89€) tells for unloaded PA6-6: E~2.5GPa, versus ~0.6GPa creep modulus for 1000h, rather good. Creep isn't linear so this modulus doesn't apply to smaller loads and longer duration.
I haven't found the creep behaviour of ETFE (Tefzel) in that booklet but generally, I wouldn't trust a fluoropolymer under lasting loads.
I've never seen an influence of humidity in the creep behaviour. Maybe there is none, or nobody has investigated, or I missed the reports. ETFE is insensitive to humidity, PA is very sensitive to humidity that makes it swell but doesn't influence much its short-time strength.
Carbon black makes miracles to protect tyres against UV, so if your ty-rap are below the thatch, it's worth trying. I hope it's your own house, not at a customer?
You might try to experiment. Load a set of ty-rap at the expected force, put them at different temperatures until they break, measure their duration, deduce the acceleration by the temperature, pretend that this acceleration applies to room temperature, deduce the duration at room temperature. That's how the reliability of electronic components is "tested". It's not really credible because the extrapolation is from one month to tens of years.
Or ask the ty-rap manufacturer rather. They may have knowledge about long-time loads. The answer would resemble: for decades, limit the force to X newton.