i expect the Charges to move back to SD is around 10 years time. Raiders lasted 13 years in LA, its not that big a deal. SD will welcome them back with open arms when the time comes.
The only reason they are moving to LA in the first place is to block the Raiders from taking over the So Cal market completely. Dumb owner over reaction. So Cal was once a 3 team market in the 80s/early 90s... granted the NFL is a different beast today.
We can assume once the Raiders sure up a long term lease in Las Vegas (minimum 30 year lease terms), then Spanos will feel less threatened.