So now it will work fine for some months, the question is how will it be in 6 months?

Answer these questions ...

1. How many miles a week do you drive?
2. Are a lot of these miles at night?
3. Are the roads flat or nearly so where you live.

Reasoning for questions ...

I repeat the car ONLY recharges the battery when coasting and that happens only lot when going down hills.
Drivng a car at night means you have the lights on ... lights use more power!
If you drive few miles from home to wherever you use a lot of juice powering up and starting the car and when you don't drive far ... enough coast-charging doesn't occur.

Note the voltage reading on the battery when you first install a fully charged battery (write it down) ... not how it drops a little lower every week?