Frequency: When necessary, really depends on how much and where you ride.
Parts: <£10 for just new inners, up to £50 for a complete set of high quality cables.
Labour: A relaxed two hours at home for a complete replacement, you'll want one of these plus an awl and some needle nosed pliers at minimum, a Dremel is nice if you want to do a really tidy job on the cut ends of the cable outers, and I like to use a torque wrench on the cable clamp bolts because the range between 'bugger, my cable has slipped' and 'bugger, my clamp bolt has stripped' isn't that big.
Frequency: When necessary, really depends on how much and where you ride.
Parts: <£10 for just new inners, up to £50 for a complete set of high quality cables.
Labour: A relaxed two hours at home for a complete replacement, you'll want one of these plus an awl and some needle nosed pliers at minimum, a Dremel is nice if you want to do a really tidy job on the cut ends of the cable outers, and I like to use a torque wrench on the cable clamp bolts because the range between 'bugger, my cable has slipped' and 'bugger, my clamp bolt has stripped' isn't that big.