Mike Shanahan recently revealed his plan for Bob Griffin's playing time this pre-season:
(5 of 16 quarters)
Game 1: 12 to 20 plays (Approximately 1 qtr)
Game 2: 2 qtrs
Game 3: 2 qtrs
Game 4: none

In case this feels like too little (or too much), here's a comparison to the 3 most recently drafted franchise QBs' pre-season playing time:

'11 – Cam Newton (Rookie, Heisman winner, 1st pick) (7 of 16 qtrs):
Game 1: 2nd + 3rd qtrs
Game 2: 1st half
Game 3: 1st 3 qtrs
Game 4: 1st drive only

'10 – Sam Bradford (Rookie, former Heisman winner, 1st pick) (5 of 16 qtrs):
Game 1: 2nd qtr
Game 2: 1st half
Game 3: 1st half
Game 4: 1st drive only

'09 – Matthew Stafford (Rookie, 1st pick) (7-1/2 qtrs):
Game 1: late1st qtr and early 2nd qtr
Game 2: 2nd + 3rd qtrs
Game 3: final 3-1/2 qtrs
Game 4: 1st qtr

IH8 pointed out to me in the past this is standard pre-season playing time for NFL starting QBs, not just rookies. But if pre-season is for (1) learning and (2) player evaluations, why don't coaches allow their starting rookie QBs to play far more than half the pre-season? After all, the coaching staff doesn't need to evaluate Rex, and evaluating Kirk Cousins at this point isn't as important as Robert gaining game experience, right?