When a stock is too expensive

 It's really hard to tell when a stock is too expensive because some stocks are expensive at a PE of 15, some others at a PE of 25 and some others at a PE of 40. 

I'm willing to pay a high price for a stock that is way above competition and that grows a lot. However, there is a limit to my will to pay a lot.

I think that my limit is something like a PE of 50 and above. 

There are some great companies that have done very well and will probably continue to do well for a long time, but their valuation is in another dimension. Just like they don't obey to the same laws of physics as other stocks.

Among them, there is Tesla, Square, The Trade Desk, IDEXX laboratories. There are also the "Pat Dorsey stocks" which are some negative earnings stocks where revenues grow a lot and should reach a breaking point soon (that way, the company will become profitable). Among these stocks, you'll find Upwork, Smartsheet and Wix, for instance. 

If you take a look at all the stocks I've written above, you'll see that they've all done pretty well over the past. But Constellation Software has done very well too at a much lower PE than many of these stocks.