It really depends on the sender. For instance, if it is a fatal event, it is unlikely that the sender will be able to resend an unacknowledged event. In general, informs are more reliable than traps, but there is no guarantee that a sender _will_ resend an unacknowledged inform, only that the sender _can_ resend an unacknowledged inform.
This is how Cisco explains informs, SNMP Inform Request:
> **SNMP Inform Request**
>
> The SNMP Inform Request feature supports sending inform requests. SNMP asynchronous notifications are usually sent as SNMP traps.
>
> Traps are less reliable than informs because an acknowledgment is not sent from the receiving end when a trap is received; however, an SNMP manager that receives an inform acknowledges the message with an SNMP response PDU. If the sender does not receive a response for an inform, the inform can be sent again.