The USSD session duration depends on the Telco.
Safaricom
A USSD session is 180 seconds long however, if a customer takes longer than 30 seconds to respond the session times out.
Airtel
A session is 180 seconds long and may be timed out if the below scenarios occur:
Application response timeout- 15s
USSD connection to application timeout- 10s
And session timeout(customer response)- 60s
Equitel
A session is 180 seconds long however, if a customer takes longer than 30 seconds to respond the session times out.
For any further clarifications send an email to smsussd@africastalking.com.