Since a Glympse is meant to be a temporary sharing event, there is currently a 4 hour limit to the initial length of a Glympse. A Glympse can be extended at any time back to the 4 hour maximum so there is effectively no limit to how long you can keep a Glympse going.
To extend an existing Glympse, go into the Recents screen in the Glympse app and add time using the Modify function. iPhone instructions here: https://glympse.zendesk.com/entries/21328284-How-to-extend-an-existing-Glympse-iPhone-
Although we currently do not currently support this feature, you can visit our Community page and vote for this new feature request.