My gas normally lasts for about 1-2 months between refills of P500. I usually put Shell Premium. I'm really having a hard time with my engine. A mechanic told me recently that this is because of the LPG that destroys the pistons. I'm not sure if he's just discouraging me with my kit or if he's really telling the truth.
In a way he is correct but only IF your LPG kit was set improperly. Taxi drivers somethings adjust the LPG settings to be too-lean, resulting in long term damage to the engine.
As long as your air-fuel ratio in LPG and gasoline are within the correct range, your engine will do fine.
BTW, I use either TOTAL PROTEC or PETRON BLAZE in my gas tank. Lasts more than four-five months and still runs smooth.