Hi, I'm from the UK and own a Range Rover Sport 2011, 3.0L TDV6 with 97,000 miles on it. I'm trying to get an idea as to how much an air strut replacement will cost parts + labor, and how long it should take at an independent garage.

I've been looking online, an air strut which includes the bag tends to be around £300 - £500, I would assume it wouldn't take more than an hour or two to replace a single air strut?

Also, is it worth replacing all air struts on all 4 corners at once for preventative maintenance, or wait until they go one by one?

Generally, it's around £1k to supply and fit a full set of 4 OEM struts

Price is dependent on whether the car has adaptive dynamics

I replaced all four struts on mine a few months back and it transformed the ride. It's easy enough to do yourself if you're handy with a spanner. Kev

MY2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
MY2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey

Remember the air strut includes the damper internally. Changing one damper is not good practice, so get a pair at very least.
At your mileage, the dampers would benefit from replacement as they will be past their best.
If you're reasonably handy with a spanner, they are not a difficult job, so why not save yourself the labour cost? Have a search for a write up, or have a look on You Tube. 2011 MY D4 HSE
Previously 2001 D2 TD5, 1996 D1 300Tdi, 1985 90 2.2 petrol.
Bikes! KTM, BMW, British, Classics and others.

How much would it cost to replace 4 air struts roughly from an indie in the UK, and how long could it talk to do them all? Is it worth doing the height sensors too

Disco_Mikey gave you a ballpark figure above and asked you if you have adaptive dynamics. This will considerably change the cost. The adaptive dampers have an electrical connector on so you can easily tell by looking at your car.

You can get all four done in a day easily. It took me about an hour per corner to do myself on my driveway.

No point in changing height sensors unless you have a specific problem in my opinion. Kev

MY2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
MY2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey