How long can you continue an exercise regiment until your body "gets used to the work out?

Your body can adjust to a routine every three to six months. Its very easy to keep your body in 'shock' so that it never adjusts though. You can shock it by training with more weight than usual; doing more reps and/or sets; doing unfamiliar exercises; and doing your exercises in an unfamiliar order. You don't have to make a completely new routine every three months.