Invest as much or as little as you like. Your percentage reward will be based on the percentage you contributed to the total pool. You can withdraw anytime with the final payment being made on Sunday.
90% of the MN rewards received will be distributed between contributors into the investment pool. The 10% deduction will covers the costs of running the node.
You can see how big the contribution pool and the pending rewards by looking at
https://www.moneybyte.org/investing/hedged-masternodes/To qualify for a share, your investment must be made by Sunday 0.00hrs GMT and will run until Saturday 23.59 gmt. (7 days).
Each Sunday the payment of your share of the MN will be paid. Please note that this will be paid in ETERN so you must have a suitable wallet to receive it.