Depends entirely on the niche and what you're using the keyphrases for. Best rule is, make it appropriate. 2-3 words is best for a top keyphrase, and 3-5 for longer phrases that are related to the main one. What are you using yours for?
u can use keyword phase according to your targeted keywords,i dont think show that is there any criteria for that.
If its too long, it's possible that Google may not recognize as a phrase. But it depends on which keyphrase they can relate to other keywords. I assume popular keywords are too saturated in SEs among websites, so naturally Google should recognize many keyphrases based on base keywords. If your keywords (base keyword) are very competitive, it's very possible that Google can recognize many keyphrases, so then they may recognize longer keyphrases too.
I think two to three words on your keyword phrase is better. Try to use also keyword suggestion tool.