Step 1:

Count number of use cases (NUC) of system

Step 2:

Set Avg. Time Test Cases(ATTC) as per test plan

Step 3:

Estimate total number of test cases (NTC)

Total number of test cases = Number of Use Cases X Avg. Test Cases per a use case

Step 4:

Set Avg. Execution Time (AET) per a test case

Step 5:

Calculate Total Execution Time (TET)

TET = Total number of test cases * AET

Step 6:

Calculate Test Case Creation Time (TCCT)

usually we will take 1.5 times of TET as TCCT

TCCT = 1.5 * TET

Step 7:

Time for Re-Test Case Execution (RTCE) this is for retesting

usually we take 0.5 times of TET

RTCE = 0.5 * TET

Step 8:

Set Report generation Time (RGT

usually we take 0.2 times of TET

RGT = 0.2 * TET

Step 9:

Set Test Environment Setup Time (TEST)

it also depends on test plan

Step 10:

Total Estimation time = TET + TCCT+ RTCE + RGT + TEST + some buffer.