The time taken by the On-demand scan is purely depend on the Amount of data in your hard disk ,s peed of the computer,what scan options are enabled etc......
i also have the same issue but i guess its due to large amount of data and my laptop is behaving very slow sometimes
There are several factors involved. As stated above, it depends on the amount of data, CPU, amount of RAM etc. I can run an ODS on a server that has 32 GB RAM and several hundred GB of data and the scan can take several hours to complete. On my laptop, I can run an ODS and it takes around an hour to complet. Depending on what settings you have configured, also affects the scanning process. Have you compared your scan time with other nodes?