The general rule is triple the weight at 8 weeks, double the weight at 12 weeks. But, as a few others have said...it is not always accuracte. At 12 weeks of age my gracie was just under 3 lbs, now at one year old she is just 3lbs 4oz.
and I should say she is healthy and round, her bone structure just didn't get much bigger.
Here is a pic of her back then and now...you can see her "size" didn't change much...
For example, MyFairLacy's weight was just over 3lbs., my girl Gracie was just under 3lbs. and their adult weight is verry different from eachothers, 7 1/2lbs compared to 3 1/4 lbs. I should also say that I used the double the weight at 12 weeks mehod, to choose my Gracie and did not get what I had expected. But I love her just as much