Which specs describes inter-working between Wi-Fi and LTE?

Question

I would like to know how long the Wi-Fi is up and when and how long it is offline.

Answer 1

Refer 3GPP TS 23.402. This specification is somewhat gives brief information of interworking of LTE and Wifi (Un-trusted non-3GPP access network). For UE handover between these two technologies few things should be done.
1. UE should have support of IP sec tunneling.
2. UE should have two IP address at a time. One IP address that is allocated by PGW during attach procedure and another is allocated by wifi network DHCP.
3. Secure tunnel is created between UE and ePDG. UE uses IP address allocated by Wifi as outer IP and IP address allocated by PGW is used as inner.
4. ePDG should have link with PGW using PMIP protocol.
5. IP address pairs used between ePDG and PGW are unknown for UE.

Reference:
http://www.slideshare.net/Netmanias/network-architecture-for-lte-and-wifi-interworking

Answer 2

TS23.234 (R 12)
TR 37.834 for the technical report of WLAN/3GPP Radio Interworking (R 12)