5.4 in Aug/Sept
Next expansion announced in Nov. (Possible Annual pass again)
Beta started in Dec/Jan
Beta will probably last 4-6 months.
New expansion releases April-June. Any later than June and we'll certainly have a 5.5 patch in early 2014.
I don't expect they'll actually meet their 18 month time table (late March), but certainly before the 2 year mark (September).