Max horizontal vent connector length depends on a few variables and may vary between the manufacturers as well. For category 1 natural draft appliances (I'm not sure what is yours) the base requirement is 75% of the height of the vertical portion of the chimney above the connector.
Based on the Gama venting tables it should be 1.5’ per inch of the vent connector’s diameter (additional requirements for connectors exceeding max permitted).
The following example is a recommendation from Energy King Coal Boiler Installation Manual:
Any horizontal runs of connector pipe should have a minimum rise of 1/2-inch per linear foot. Use extra support hangers or brackets every three feet if it is absolutely necessary to have a run of more than six feet, which is not recommended.
Maybe if you give me some details about your boiler (manufacturer, model) I’ll be able to find more precise answer. Just click "comment" below.