What Is Correct Dosage Of Taking Valtrax?

Hello i just had a question about taking valtrax. Is there a specific dose you have to take for it to be an overdose. I got a cold sore recently and I've been taking valtrax. The doctor told me to take 2G in the morning and 2G at night to kill the virus and so i did but i also took 1G the next day. Is that too much to be taking? and also will my body grow a tolerance to the pills so that the next time i get a cold sore i'd have to up my dose? or do viruses not get immune to antivirals Thank you