Whats too much leverage?

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Hi all,

I wanted to pick everyone's brains a bit on leverage and how much leverage is too much leverage. Currently I own 5 rental units. One property is fully paid off and the second I only owe about $4,000. I have about $110,000 of equity based on the properties value. All together I owe about $52,000. Each one of the properties cash flow about $600-700. I have been funneling the cash flow into principal payments in order to pay off the homes quicker and pay less interest. 

My question is... instead of making principal payments towards the homes should I be using the cash flow to purchase even more homes? Therefore, how much do you all think is too much leverage?