The 40 credits thing simply determines eligibility. So, you would be eligible if you earned 40 or more credits. More than 40 doesn't change anything.
Eligible just means you get a check, it doesn't have a bearing on how much that check is for.
For example, let's assume further that you worked for 10 years (minimum to get 40 credits) and earned the average US wage during each of those years. For example, these wage values per year:
Then at age 67, you would receive a check for $920 per month in 2019, and this would get adjusted upwards for inflation every year thereafter. This would be true if you earned the average wage in any other set of 10 years.
If instead, you had earned the average wage for 20 years, in 2019 at age 67 your monthly benefit would be $1,347.
So, the answer depends on what you mean by "full benefit".
A useful online tool you can use to play with these calculations and see how it effects your benefit can be found at https://socialsecurity.tools/ I used it to produce these benefit estimates.