Here's a rundown of what Tapas milestones do:
25 - Actually doesn't do anything, it's just to make the 100 milestone seem less intimidating for newbies.
100 - Unlocks ads and ad revenue.
250 - Unlocks the ability to receive ink donations and to withdraw earnings from ink and ads when you have $25 or more of cleared funds (funds clear in 30 days).
There is no automated means to change your comic/novel to "wait for free" or "early access" on Tapas. You pretty much just have to try to accumulate a big, engaged audience and hopefully the staff will notice the high performance and offer this to you.
There have been mumblings about a change to the system, like in a Bleeding Cool article a month or two ago, but they may have just been rumours or leaks of early plans that didn't come to fruition.
