A session in this context is one continuous TCP connection. If the session was authenticated using OAuth credentials, it's limited to about the validity period of the access token used (usually 1 hour).

Gmail IMAP sessions are limited to about 24 hours. Gmail POP sessions are limited to about 7 days. If your client begins with plain text, before issuing the STARTTLS command, use port 465 (for SSL), or port 587 (for TLS). The outgoing SMTP server,, supports TLS. Incoming connections to the IMAP server at :993 and the POP server at :995 require SSL. The SASL XOAUTH2 protocol documentation describes the SASL XOAUTH2 mechanism in great detail, and libraries and samples which have implemented the protocol are available. The SASL XOAUTH2 mechanism enables clients to provide OAuth 2.0 credentials for authentication. IMAP, POP, and SMTP use the standard Simple Authentication and Security Layer (SASL), using the built-in the native IMAP AUTHENTICATE, POP AUTH, and SMTP AUTH commands, to authenticate users. The Gmail IMAP, POP, and SMTP servers have been extended to support authorization via the industry-standard OAuth 2.0 protocol. For non-Gmail clients, Gmail supports the standard IMAP, POP, and SMTP protocols.

Thunderbird gmail oauth2