See TN199812A for general information on the GPS system.
This Tech Note will hopefully explain why you have to wait so long for the UTC offset.
The time used by the GPS system does not include leap seconds. These means that every eighteen months or so, its "time" changes by one second with respect to UTC which does include leap seconds. The difference between GPS time and UTC is 13 seconds in early 1999 and is referred to as the UTC offset. This value is obtained by the GPS receiver from the satellites in a "navigation message".
The navigation message is transmitted by the satellite on the L1 data link at a rate of 50 bits per second. The message structure uses a basic format of a 1500 bit long frame (30 seconds) made up of the five subframes mentioned in TN199812A with each subframe being 30 bits long. Subframes 4 and 5 are repeated 25 times each, so that a complete message will require the transmission of 25 full frames (12 1/2 minutes). The UTC offset is only transmitted once per frame, so it is only transmitted once every 12 1/2 minutes. Depending where you were in the cycle when the GPS receiver is first switched on, it can thus be up to 12 1/2 minutes until the UTC offset value is transmitted from a satellite.