I expect the 13" rMBP to come out at around the same time as the price of the 15" is reduced and the cMBPs are discontinued. Probably in time for Haswell.
We would have something like:
MacBook Pro 15" (Mid-2013)
2880x1800 Retina Display
Quad-core 45W Haswell Core i7
8GB RAM (16GB Option)
Nvidia GT750M / AMD Radeon HD 8770M
256GB SSD (512GB+ options)
$1999
MacBook Pro 13" (Mid-2013)
2560x1600 Retina Display
Dual or Quad core 35W Haswell Core i5/i7
8GB RAM (16GB option)
Intel HD 5000 (Twice as fast as HD 4000)
256GB SSD (512GB+ options)
$1499
Then in 2014 there would be an update to Broadwell and new GPUs and probably another $100-200 price drop to bring back the prices closer to those of the previous cMBPs.
After that, in 2015, 512GB SSD would come standard at the same prices. In 2016, there would be more minor specs bump (simple CPU/GPU generation change, still 8GB RAM) and at that point rumors of a redesign will have started and some people will lose interest in the 13" rMBP and move towards the new 13" rMBA which would be thinner and lighter and not significantly slower than the 13" rMBP.