Strings should just be replaced when they "go dead", which is typically in 3 to 6 months, depending on the string and the amount of use. It is also wise to change your strings when they break. :-)
You can clean your strings with alcohol by applying it to a cloth and rubbing it on the strings. The best solution to gritty strings is to be sure and wipe the rosin off your strings after each use.
Todd French