1. Is it possible to know how much is the time limit of a access token for a connected Org
Answer is No except you hit salesforce endpoint using access token and if you get 4xx as response it means token got expired and you can call refresh token to get new token.
2. Forcefully expire token
To revoke OAuth 2.0 tokens (access/refresh), use the revocation endpoint.
Construct a POST request that includes the following parameters using the application/x-www-form-urlencoded format in the HTTP request entity-body. For example:
POST /revoke HTTP/1.1
If an access token is included, Salesforce invalidates it and revokes the token. If a refresh token is included, Salesforce revokes it and any associated access tokens.
You can also invoke using GET request with parameter token