It depends on the people invovled and the situation.
Sometimes an ex wants more than friendship but is willing to "settle" for it with the hope of one day starting over again. Beware of the "instant friendship"
Generally speaking it's rare to go from being "red hot lovers" to plantonic friends overnight. What usually happens is there is a gap of time where there is little if any contact and one day you bump into one another after you have both moved on. There is some polite conversation and "gradually" a friendship develops.
You don't want to become to emotionally invested in a friendship with an ex because things can change dramatically once one or both of you become involved with other people. If a person is "uncomfortable" with her or his mate being "friends" with their ex it's not uncommon for the ex to create some distance in order to maintain their happy relationship/marriage. Love trumps friendship most of the time!