The dullness and weight you're describing usually means product buildup, so clarifying shampoo can definitely help reset things. The tricky part is finding the sweet spot - use it too often and yeah, you'll dry yourself out, but used right it's actually pretty gentle. Start with once every two weeks and see how your hair responds over a month. If it feels better and bouncier, stick with that. If it still feels heavy, you could try bumping it to weekly for a bit, then back off once things improve.
What matters more than the frequency is what you do after. Clarifying shampoo strips everything, so follow up with a decent conditioner - don't skip that step or you'll end up with straw. Also think about whether you're using a ton of leave-in products or styling stuff, because that's usually what causes the buildup in the first place. If you're piling on serums, pomades, or heavy conditioners every day, even clarifying once a week won't fix the problem long-term. Sometimes it's worth cutting back on daily products rather than relying on the clarifying shampoo to do all the work.
And honestly, if after a few weeks your hair still feels dull even with clarifying shampoo in the mix, it might be worth checking if your regular shampoo is the culprit. Some drugstore ones leave residue no matter what. But start with the clarifying approach first - it's cheaper than replacing everything.