On how many systems can I install the s/w using one license and how long can I use the s/w?

You can use an ETS Question Bank on any machine you want. The same license key will work. However, only you are the licensed user and not anybody else. Having other people use your license key would not only be unfair to us but would also be illegal.
You can use the s/w as long as you need. There is no time limit. However, free upgrades and support are provided only up to 6 months after purchase.

Note: Access to Web/Android version is on monthly basis because it requires continuous access to our servers to store your data.