in lib/rails/generators/mongoid/config/templates/mongoid.yml

1. The timeout to wait to execute operations on a socket before raising an error.
2. (default: 5)
3. socket_timeout: 5

in docs/reference/configuration.txt

1. The timeout to wait to execute operations on a socket before raising an error.
2. (default: nil)
   socket_timeout: 5

It appears the latter is correct–the default socket timeout is nil.

As a real-world mongo user, I would suggest we set the default to 5 seconds in the next major release (Mongoid 8.0) Something conservative like 30 seconds would make sense to add as a default now.

Given that large queries are done using GET_MORE cursors (which will not be subject to the timeout as long as each iteration is under X sec), there is really no practical case where you'd want to set it to something higher than 30 seconds; it typically means your queries are unindexed and you're going to kill your database.

Don't ask me how I know