I understand your concerns entirely since we went through the same thing. Our septic was inspected just prior to closing and "passed" although I think in our locality, the inspection consists of running some water and checking the level in the tank before pumping, i.e. the very basics. All was well until one snowy winter day 6 months after we moved in when the septic backed up into our basement bathroom. The plumber came, opened up the cleanout (which is inside our house!) and out poured gallons and gallons of septic water! He tried to snake the line but it was still backed up. Managed to find a septic guy to come out on a sunday to pump. He could not find any thing wrong with the septic and though perhaps some toilet paper had blocked the outlet. Four more months passed before our 2nd back up, followed 2 weeks later by our third. Called in the experts again and this time they discovered that the leach lines had collapsed and somehow we had survived almost a year on about 6 feet of leach line (should have been 200 feet). Long story short, we had to shell out $4300 for a new leach field (thank god for my generous parents who helped us out).
The septic guys told me that leach fields last between 20-40 yrs depending on how well the system is maintained and construction. Our house is 43 yrds old and I believe that it was the original system and was therefore at the end of its natural life. Wish I had known all this when I was buying the house. I'm not sure if anyone can really tell you how much longer you have, it will depend a lot on how you maintain it and how much water you use.
Hope this helps