Assuming you have the OVA (> 6 GB for ISE 2.1), it takes about 15-20 minutes to deploy into an ESXi host assuming a fast (local) network connection.
You then need to run the setup bootstrap steps (about 5 minutes to enter values and then 20-30 minutes will the application installs).
Add the certificate(s) requires a web server restart - under 10 minutes.
Finally join it into the deployment - requires application server restart - about 10-15 minutes.
So about 1-1/2 hours if you are familiar with the steps and all goes well.
Could be longer if you are not.. :)
...or if your deployment is running a patch level beyond the base OVA image available as you'd need to install that as well before being joined into the deployment.