You should replace parts based on wear, not time.
Tires: when the treads are worn away (or sooner if damaged or sliced or dry-rotted). This may be after one, two, or several years. Or this may be after 3 months. The determining factor is wear, not time. Tire composition is important, too. My "race-day" tires are only good for about 1000 miles. My "training" tires are good for about 2500 miles. Rear tires wear out in about 1/2 the miles as front tires.
Chains: when the chain has stretched from use. "Stretch" is actually roller wear which is determined by carefully measuring the distance between the pins 12 links apart. That distance should be exactly 12 inches. Manufacturers recommend replacing a chain which has stretched 1/8" between 12 links. But many experienced cyclists replace at 1/16". Waiting to 1/8" can result in cassette damage requiring cassette replacement in addition to chain replacement. Chain stretch primarily depends on mileage ridden, and on how clean you keep your chain. Riding a gritty, dirty chain accelerates wear. Replacing your chain--or any other part-- based on a calendar ("twice a year", or other such time-based schedule) is completely uneccessary and a waste of $$. Ultra-narrow chains have been known to last 1000 miles or less. Some chains can last 5 times that distance.
Cassettes: when the teeth on the cogs you use most are noticeably worn, or if you have shifting problems that don't go away, despite replacing your chain and despite all attempts to properly adjust your shifting cable, you probably need a new cassette. As in chain use, this is based on usage and wear, not on time. Typically, if chains are replaced in a timely fashion, you will replace several chains before having to replace a cassette.
Chainrings: see "Cassettes" above. Typically, if chains and cassettes are replaced in a timely fashion, you will replace several cassettes before having to replace your chainrings. Once again, think "wear", not "time".
Cables: Replace immediately if frayed or kinked. Otherwise, replace if noticeably corroded, or if shifting is spotty even after replacing chain and/or cassette. Depending on riding conditions (rain/wet) and mileage, a one-year, or a two-year replacement schedule for shifter cables is a good idea. (I realize that this is a time-based replacement schedule for cables, in addition to a wear-based schedule. But even good-quality cables are inexpensive ($3.00--$5.00 apiece), so this would not be a waste of $$.
Cable-housing: Replace immediately if kinked. Otherwise, replace if new cables do not slide smoothly within housing. Once again, housing is relatively inexpensive, so replacing housing every other cable-change would not be a waste of $$.
Handlebar tape: Replace when worn or torn. Use-based, not time-based.
On the above parts you should pretty much go by the "if it ain't broke, don't replace it" philosophy. Regular care and maintenance will extend the time before replacement is needed.
Bob