How long to get the response for REF at TSC?

The USCIS email said they received mysupplemental documents for REF on Dec20,2005 But it's been more than 3 months, I still haven't got any further information. What's normal time frame I can expect from USCIS about the decision after submitting response to REF? Anybody has the similar experience? Your comment or experience is highly appreciated. BTW,it is TSC. Also, should I call national center to check the case status? Thank you so much!!!