Hello,
>1. How long such a certificate will be valid?
Let's Encrypt is issued for 3 months. When this period is over, your certificate will be renewed automatically
>2. What kinds of domains will be validated (EV, OV)?
Only domain validation (DV) is supported
>3. Does it support multiple subdomains?
Unfortunately, no. You will need to a separate certificate for every subdomain.
Please, also refer to our Documentation for more info
http://doc.ispsystem.com/index.php/I...80%99s_Encrypt