Does index rebuild time depend on the fragmentation level?
For everyone interested, I have created a chart showing the index REBUILD duration of about 2500 index rebuilds within couple of weeks in relation to the fragmentation of the index and it's size in pages. 

This data is based on 10 SQL Servers, hundreads of tables and on Ola Hallengren's optimizing procedures. The general threshold for rebuilding is set to 5% fragmentation.

I have cut off some of the the largest tables (10 Mi + Pages) in this statistics to make it more readable.

The chart shows the required time (duration) as size of the bubbles. The biggest bubble's values are about 220 seconds.
It shows that the required time to rebuild an index is not really related to the fragmentation. Instead it seems to be more depending on the number of pages the index has. Also it indicates that low-level fragmentation is more time-consuming than higher fragmentaion.


The second chart is just zoomed into the area &lt;= 200 K Pages. It shows the same, it takes longer for larger indexes, not for more fragmentation.