Connection time is airport dependent -- things to consider
-- Gate /terminal layout
-- frequency of weather / airport delays
-- your mobility ability
-- where you might be sitting on the arrival plane
-- your desire make use of time on the ground (food, refill water bottles, restrooms, ...)
and your sensitivity to missing a connection -- what options exists, if you miss your scheduled connection

30-40 minutes is very doable much of the time -- but it will require risk tolerance. Generally 60-90 minutes is better for most folks.

On the UA forum we have some airport specific treads for all the UA hubs and some other airports
http://www.flyertalk.com/forum/unite...ional-ewr.html
http://www.flyertalk.com/forum/unite...ional-iad.html
http://www.flyertalk.com/forum/unite...ional-ord.html
http://www.flyertalk.com/forum/unite...ional-iah.html
http://www.flyertalk.com/forum/unite...ional-den.html
http://www.flyertalk.com/forum/unite...ional-sfo.html
http://www.flyertalk.com/forum/unite...ional-lax.html