Good wool is very durable, so with 'normal' wear it will last a long time. The biggest threat is moths!

You shouldn't have to clean a kilt very often, especially if you spot-clean any spills as soon as possible. Wearing underwear also helps keep it clean! But of course, that's a personal choice.

A good way to clean a lightly-soiled kilt is with a professional-type steamer, plus spot-cleaning of any stains before steaming. The method of spot-cleaning depends upon the spot. When steaming, use pins or clothespins to hold the pleats in place to maintain the crisp pleating. You may need to press pleats on a flat surface with a steam iron and pressing cloth, too.

If you have a kilt made, ask the kiltmaker to send you any scraps that may be left over (it can be a meter or more if you bought a special-weave tartan; otherwise, probably not very much at all). Save them to use for testing cleaning products. You can also test on the inside apron, if you have no scraps to experiment on, but be very careful!

Here's what Matt says in the Scottish Tartans Museum web site, about hand-washing a kilt:

CLEANING: Most say to dry clean your kilt only, and this is certainly a safe bet. However, if you wish to wash your kilt yourself, here is the recommended method. Spot the cloth by using undiluted Woolite (or equivalent) on the especially soiled areas and rub thoroughly. Fill your bathtub with 6 inches of cold water and 12 capfuls of Woolite. Swish your kilt around in the water, soaking the entire garment. Fold the kilt as it is worn, then lay it front down in the soapy water. Soak for an hour. Then hold it over the tub letting the dirty water drain off. Now hang your kilt on the clothesline and soak with a garden hose to rinse. Take care to make sure all soap is removed, especially between the pleats. Let it drip dry, out of direct sunlight. Your pleats should not lose all of their press, but you may repress with a steam iron and a pressing cloth.

Don't wash a woollen kilt in hot water, nor in an automatic washer, nor dry it in an automatic clothes dryer.

I think it's great to wear your woollen kilt other than for formal occasions. I believe you'll find that with reasonable care, it will give excellent service and outlast all your other clothes.