If you have no modified any core code and all your plugins have been updated to work on 1.7 (they use action tokens), the upgrade should not require that much time. Make sure you set up a test site first.
If you have modified the code, you will want to do a diff between your current version and Elgg 1.5 to identify the changes and then port those changes (hopefully as plugins rather than core hacks) to 1.7. The changelog for 1.5, 1.6, 1.6.1, 1.7, and 1.7.1 will not help you as there have been thousands of changes. Better to move your changes to 1.7.
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