Question: Steady state concentration


I'm new in Maple and i've a project to do. I've to find at what time is 90% of the steady-state concentration of C achieved. And the eq is 

z'=k(1/2-z)(1/3-z), where k=0.3. 

So i had worked in Mathematica before and i know that it would look something like that in there: 

eqn1 = 0 == k (1/2 - z[t]) (1/3 - z[t])

DSolve[Evaluate[{eqn1}], z[t]].

But i have no idea how to find this 90%. I hope that someone will be able to help me somehow and i'm not a big fan on mathematica and differential eq so please keep it simple ;D

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