In the metro and RER, tickets are valid for two hours for a single, one-way trip, which might include transfers. It cannot be used for several different trips within this time period.
In the bus network, tickets are valid for 90 minutes between the first and last validation, including one or several transfers. T+ tickets must be validated each time you board a bus or tram. Bus drivers also sell tickets, but these are only valid for a single trip, without transfers.