Time-based tickets allow an unlimited number of transfers within the validity period, but must be validated at each boarding. A journey started within the validity period can be completed on the vehicle being used even if the time limit has expired.
For example, with a 30-minute ticket you can board until the 59th second of the 29th minute, while with a 90-minute ticket, your last boarding must be done by the 59th second of the 89th minute.