Purchasing More Investment Homes Strategies

So i bought my first investment home this past month.

I bought it for 330,000.  I am making 4200$ a month my mortgage payment is 2500 a month.  So, im about 1700 cash flow a month right now.

I do have some extra money saved up, however i do have a 300,000 loan on the house i just bought but the mortgage is being paid off with the rental income.

I am wondering when is it smart to purchase my next investment home.

Also the next home i purchase is it fine if the rental income just pays off the mortgage or am i only looking for strictly postitve cash flow properties.  

I am wondering should i sit and wait for a good deal ?  I dont want my money just sitting in the bank.   I do have a full time job where i make about 5k monthly as well.

I am 25 years old i want to get as many multifamily properties as possible i just want to know if its smart to do it and what are some of your experiences and strategies.