The length of a ban resulting from a red card is at the discretion of the governing body of that match, under the guidance of the International Rugby Board(iRB) Regulations 17 to 20.
Typically the accused player will front a judiciary and both his admission of guilt and the seriousness of the offence will be taken into account when the length of a ban is considered.
It is entirely possible that a player, found guilty, will receive no ban and be able to play the next match. An example of such a situation from the 2011 World Cup is Samoan fullback Paul Williams.