How much cash flow is enough? Boston

[Chris Roche]

So I have a few questions.  Any help would be greatly appreciated.

I am looking at multi-families, putting for 20-25% to buy as an investment to rent out.  Doing the math on the mortgage compared to the rents, I would only be making a few 100 a month.  With a down payment of 25% at 100k, that would take far too long to recoup. 

How much money is a good amount to make on a rental per month? I understand its a long-term process but I don't want to take years to pay off just my down payment. 

How can you find a good deal to make that amount?  Obviously buy as low as possible, but everything is high around the West Rox, Rosi area.