I have written about the main factors to be considered before producing an effort estimate for system testing. These are:
a. What is the size of the system?
b. What types of testing is required?
c. Whether only scripted testing or only exploratory testing or their combination will be performed?
d. What "non-testing" activities would be performed? Examples include defect reporting and tracking, production of reports and project management.
e. What is the estimated number of test cycles that would be performed?
You can reach the complete article
here . The article also mentions a number of approaches for test effort estimation.
Inder P Singh