How long should a leather strap last?

As above. I bought an Omega leather deployant strap when I got my Speedmaster '57 in November 2017. Pretty sure I paid £160 for the strap alone. I use the watch in rotation with one other and I rotate the strap and the bracelet on the '57. I suppose at most it’s seen 6 months use. Furthermore, during those 6 months use, it is only worn at odd evenings when I’m indoors and weekends. Yesterday I removed the deployant clasp to fit it on to another watch strap and I noticed that the ends were fraying/splitting where the deployant spring bar goes through it. I'd say there is now only 80% left of the part of the strap that the spring bar locates. If that goes, I could potentially lose or damage the watch. The lug ends of both strap halves are perfect. Is this normal wear and tear?

Pictures? I’d say that the damage was unusual. A quality deployant strap should last years when worn in rotation with other watches
Do you wear the band very tightly? Submerge it in water? Use leather conditioners?

Many have offered advice above, but I'll add.

Much depends on the construction and materials, as well as how much oil / sweat is being absorbed by the leather. If as you note, the lug bars are showing signs of stress, that tells me there might be an issue with the material thickness there.

I had / have a DiModell strap on another watch, which I assumed was 100% leather. The material closest to my wrist was a good synthetic 'leather' which after about a year was flaking and shedding. It didn't affect the lug bar area, but it wasn't exactly confidence-inspiring. A good leather strap can be really dressy, but I prefer a bracelet since I don't like worrying about a strap wearing out or failing. YMMV.

Your storage environment makes a big difference. Dry is good for the watch but killer on the strap. Unfortunately, you do not know how it was stored at the store. Are you in an arid environment? Make sure environmental humidity is not low, and if it is, get a humidifier designed for wooden instruments.

You can also treat your straps to make sure the oils within the leather do not dry out as well. I prefer Allen Edmonds Leather Lotion but other similar products should work fine. Perhaps this:

https://smile.amazon.com/Cadillac-Leather-Conditioner-Handbags-Furniture/dp/B012O1XJNC/ref=sr_1_23

Tom

A leather conditioner or lotion would help, but it depends. Are you in and out of water? Is the environment very dry or wet where you are? Like any leather good, there are many factors. Keep it relatively dry and conditioned and you should be ok. That said, a little fraying gives the strap a bit of character as long as it doesn't degrade too much or hurt you in some way.

I bought a quartz Seiko for my 21st with money I was given. It came on a bracelet but I soon changed it for a leather strap. It must have lasted me around 4-5 years of daily wear as I remember replacing it a few (4-5) years later. I still have it and it’s still on leather - probably about 4 straps later!