Well, it's up to you to figure it out based on your card pool, really.
However, some numbers are handy:
- At the start of the game, your opponent will have at least 33 cards in his or her deck (more if there are mulligans).
- Each instance of the "mill until you hit a land card" ability will mill something like 2.5 cards on average. That's pretty weak.
What Dimir does have going for it is that a lot of the milling in this set is attached to other stuff -- the common Balustrade Spy is a feeble mill card but also gives you a 2/3 flyer for a reasonable cost, for instance. And you do have access to some repeatable milling, like the prerelease promo Consuming Aberration and lower-rarity carsd like Paranoid Delusions, Sage's Row Denizen, Undercity Informer, and Duskmantle Guildmage.
I don't think Gatecrash's milling is nearly as explosive as similar decks in Dark Ascension Limited. And Dimir creatures are by far the weakest of all the guilds; if you want to actually play a solid defense, you're likely to need Orzhov or Simic to help you stabilize (Dimir can work well with both those guilds because it offers them the missing component they need to be a control deck -- lots more removal for Simic, countermagic for Orzhov). If I'm mixing guilds like this, though, I'd much rather be trying to win off of cipher+extort than straight-up milling.