The expected service life of any engine depends on how you use it...
For a sport flyer, using 10% nitro fuel, and the .40 ci class engine which is properly tuned, going through a gallon of fuel a month: The LA engines could last 3 to 4 years (or more....), barring a crash damaging it. The Magnum XL, about 4 to 5 years, the TT Pro, 4 to 6 years, the O.S. FX 5 to 7 years, but could last more than 10 years. (That's a lot of use even with the LA...)

You will see an improvement in how long the engine lasts by buying the more expensive engine. (assuming its properly tuned, and not crash damaged.) You do get decent value for the dollar with the more expensive FX engine.

However... the peak power life of each engine will be much shorter than the total useable life. The first 1 to 3 gallons of fuel, the engine is breaking in. The next 5 to 10 gallons is peak power output, with a gradual decline to where the engine has worn out the piston and liner. (the bearings usually go first in the LA, and other bushing type engines.)

As an example of the possibility... I have an exceptionally good O.S. .25 SF series engine (similar to the FX in many ways) which is over 17 years old, which has had over 35 gallons (I lost track long ago... might be 50 gallons... ) of fuel run through it, (appx 20 min for each 8 oz) and still produces almost as much power as a new .40 LA. I've also abused engines (extreme nitro fuels and running hot at high rpm...) and ruined them in less than 6 months.... No manufacturer should be expected to cover crash damage or abuseive operation.