From FCTM "landing with abnormal landing gear":
Considering a realistic hydraulic demand, the hydraulic power remains available up to approximately 30 s after the shut down of the related engine
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As a reverse-engineering method to check how much N2 do you need to get hydraulic pressure, as setting the parking brake after pushback will deplete some pressure from the accumulator, if you don't recharge the accumulator and start No2 keep an eye on the triple indicator, notice when it moves up and then check N2. Now you will know. If I'm not mistaken, N2 above 10-ish will give pressure