In order to be able to give you an exact number, we need to know the following information:
How big the data that you're encrypting/decrypting
Computer specifications (OS, processor, RAM, etc.)
Other factors like programming language used etc.
The average time for encrypting/decrypting 3mb file is about 1.5 sec. It can be different when you work with streams and takes like 1 sec or even less.