Eh!
Who knows!
There's so many variables.
Is the grassy hill dry, damp or boggy?
How much of a run up can get?
How high is the hill?
How much tread is left on the road tyres?
What size wheels are you running?
Straight answer :- You just keep trying till you bottle it, or you burn the clutch out or you roll over backwards! simple as that.
Jeremy Clarkson drove up sodding great a mountain on road tyres!
That's nearly as daft a question as "How far can you drive on a single sump of oil"
Come on... stretch the boundries here Ben.... !