Test Estimation Tips

• Have a deep understanding of the scope of the project. List down deliverable's and identify the types of testing required for each deliverable. This would be your base document. 
• Identify at very initially if performance testing or security testing is required. Effort estimation for these should be done a separate way.
• For each deliverable, identify the test environment. It might be common for all or may be two or more – depending upon the nature of project or modules.
• Against each deliverable, identify the test case documents, scripts to test data to prepare. Do not miss to include review & rework effort of each of these.
• Try to have a brain storming session with BA, testing team, development team & drill down each deliverable to feature / function level. If time permits, you can do this exercise formally & create a detailed WBS for this containing testing points / scenarios.
• Generally, time does not permits for this. So, basis discussion with BA, you can do some rough work on your notepad & create some basis for estimation.
• Include bug life cycle. Mostly, we miss the effort that does into bug triage meetings & effort goes into defect management. 
• Make sure to factor-in the availability of resources.
• Remember, doubling the resources does not necessarily means that effort will be reduced to half. Evaluate what testing can be done in parallel.
• If you are working for the same client or with same development team, consider your past experience. If you know testing team & development team members, you can do better estimation.

Always, publish the estimation with some assumptions like

• This is the estimated effort assuming that 4 senior test engineers will work on the project
• This estimated schedule may change if testing team receive continuous unstable builds. However, testing effort would remain same with +5%
• It includes effort of complete functional & usability testing.
• Regression testing effort will be estimated after completion of first testing cycle and after evaluating the bugs found in first cycle.

Last, but not the least, always add some buffer in your estimate.

source: http://www.softwaretestingstuff.com/2013/01/points-to-take-care-while-doing-testing.html