MTBF is published on the spec sheet for branch SRX
http://www.juniper.net/assets/us/en/local/pdf/datasheets/1000281-en.pdf
But not on the Data Center line.
https://www.juniper.net/assets/fr/fr/local/pdf/datasheets/1000254-en.pdf
Since they share some of the base chassis components I suspect they are only available via NDA like the MX ones.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB11145