Hi Pavlos.
I have been on Metformin for many years before finding a way to control my DIabetes. I was on the maximum doseage of 3g daily. It did some work towards controlling my Bg levels but I soon found after coming here that it was not the answer to my problems. My diet, my lifestyle was. That is well documented elsewhere and I don't need to go back over it.
Because of the better control I now no longer take Metformin at all, or Gliclazide - all gone. I do take Byetta though, so not all meds are finished. This drug in particular is far better for me than Insulin as it doesn't have the same problems with possible weight gain.
Using oral meds does not mean that you will automatically progress to Insulin. Only if you are not well controlled do things need to be treated, by whatever means. There are other alternatives to Insulin such as Byetta, Victoza etc. There are members on here who have successfully controlled their diabetes for years by diet alone. You can read about them on here as well.
The known side effects of Metformin are as follows: (They are not mandatory BTW)
Anorexia, nausea, vomiting, diarrhoea (usually transient), abdominal pain, taste disturbance, rarely lactic acidosis (withdraw treatment), decreased vitamin-B12 absorption, erythema, pruritus and urticaria; hepatitis also reported.
Your B12 should be monitored every 6 months as well.
If you have no side effects and it is working I shouldn't worry too much. I never had a single side effect even when on the max dose. Something to do with drinking Guinness over many years and a cast iron constitution.
So, you do NOT have to stay on Metformin for life, however it also has other beneficial effects which you may want to consider. Discuss with your Endo.