I bought a brand new set in Feb last year. Reasonable mid range ones. Cost me c£200 for the whole lot.
Having bought a set before and never owning a car so long ( 20 months), how long should a reasonable set last me 'til I need to get a new set.
I do drive quite ‘heavily’. 
About 8k miles p.a.
Not a direct answer to your question, but just reminded me of advice that is given to caravanners
We are told to replace our tyres after 5 years because of U.V. damage weakening the tyres, and also to move the caravan occasionaly so its not sat on the same piece of rubber for long periods - I would suggest this is good advice also for anybody who has vehicles off road for long periods, or low miles P.A., or are just very careful drivers.
So Pat even of they are good after 5 years, perhaps time to for a change
This is a bit like how long is a piece of string isn’t it?
It all depends on how you drive (hard acceleration, hard braking, hard cornering etc) and what mileage your doing. Also whether you have the wheels aligned and balanced properly.
For a FWD car, in my experience a good set of tyres with the car adjusted properly with medium (not hard) driving will last around 25,000 miles for driven wheels (fronts) and 30,000 for non-driven wheels (rears)
For RWD (ala MX5) then with carefull driving I have got 30,000 for all wheels (not on a 5 though - Ford Sierra). Of course this will vary with the tyre as some have softer rubber than others. I would guess that you can half this if you drive harshly or don’t have the car set properly.
When I bought my 5 privately the orginal tyres (Bridgestones) were just about worn out at 24,000. The way I drive I expect to better that on the new set I’ve put on.
It is also true that tyres deteriorate with age, although I have some on my Ford Focus now that are 5 years old and they are fine (no cracking etc). I would carefully inspect any that are more than 5 years old even if they have not been used much.
I agree with Phil. The original Bridgestones on mine were replaced like for like @ 25,000 miles. The replacements have done about 10.000 miles & ive been impressed how theyve performed in this snowy / icy weather.
I got about the same mileage on my Astra & Zafira which had Goodyears IIRC
Depends entire on how you drive. I can take a new pair of tyres down to the canvas in a couple of hours on a track [:D]
Just check your tyre tread depth once a month and it’ll give you an idea of how quickly they are wearing out, also it’ll help you spot any alignment problems you have too.
I just replaced my 215x40x17 ContiSport Contact 3’s after 11,000 kilometres … just under 7,000 miles. I could have swapped them around again to get the last poofteenth but it wasn’t really worth it. Two were just about at the wear limit and two a couple of mills above. I’d rather get a set at a time. Wear was even and no track days etc. They were great, with the emphasis on “were” (wear factor 280).