Any noble may seek the hospitality of any other noble. To do so they present themselves to the master of the house, and ask for hospitality. Failure to extend hospitality to a fellow noble is a serious faux-pas.
Such hospitality includes both room and board. The kind of hospitality extended depends on the relative status of the guest - knights visiting a baron may be seated with his knight-bachelors and housed in the common room, a fellow noble would be offered a seat near the baron and private quarters, while his liege lord would probably be given the quarters and seat of the baron.
Guests are expected to behave cordially, and to share news, songs and tales. If their host has need of bellatores during their visit, they should assist him, unless this would clearly conflict with other loyalties, in which case they should remain utterly neutral while under his roof.
It is considered somewhat boorish to deliberately stay more than three days with one host, unless asked to (see below). Furthermore, it is considered to be good manners to offer your host a gift upon coming or going, particularily if you are not in a position to reprociate the hospitality anytime soon.
To leave a host, one must present oneself to him and ask his permission to leave. A host has the right to insist that any guest stays - no one may leave a host without his express permission. It is considered to be verging on treason to insist that one's liege stays longer than he desires to.
Guildsmen tend to extend hospitality to fellow guild members. Such hospitality is less formal than that of nobles, and guests are treated as friends or relatives (often they are).
There is no clear-cut rule as to when guild hospitality expires. Sometimes, guildsmen are permitted to stay indefinitely, provided they share techniques and lend a hand at work.
Often, travellers will find themselves without access to inns or obliged hosts. However, pretty much any common farmstead will accept guests for a sum of $1-$5, less if they have news, tales or songs to share. However, this hospitality is only good for a single night.