Hey Dipu,
1. Make sure you have all your workflows you will need created and enabled.
2. Upload 100mb(can be more) of assets and measure time it will take for your instance to process them.
Check the time start = when you start your upload, end = when all the workflows have finished running in your instance.
This will give you an idea how long it takes to process 100mb of assets, then you can multiply the time with the time of your total assets.
Hope it helps!
Regards,
Peter