Mine goes on when I get up and is at about 20% come bedtime. Not sure when I’m meant to charge it!
Which generation Apple Watch do you have? This sounds like there is something wrong with your battery. The battery should easily last you a full day and even two when you don't do anything that could drain the battery faster than usual.
I would take it to Apple for a battery replacement. And if you live in the EU and you also bought your Watch in the EU, don't listen to what Apple is saying about warranty. European consumer laws state that you have the right to a decent product, including a decent battery. That means a battery should be able to retain at least 80% of it's original capacity for at least 2 years. Especially for a watch.
If the battery breaks down in the first 6 months, it's up to Apple to proof that it broke down because of abuse. If they can't they should repair your watch free of charge.
If the battery breaks down in the second 6 months, it's up to you to proof that you didn't abuse the battery. If you can, Apple should also repair your watch free of charge.
If the battery breaks down after the first year and you can proof that you didn't abuse the battery, you shouldn't have to pay the full price for the repair either. Instead you only have to pay the percentage of the costs relative to the expected lifespan of the product. In this case, when the battery broke down after 1 and a half year, you only have to pay 3/4 of the repair costs, as a battery is expected to last at least two years.
On their website, Apple states that the AppleCare and AppleCare+ are an addition to the European consumer laws and not a replacement. But in the shops, Apple's personnel usually refer to AppleCare as your only warranty and just tell you that you have to pay the full price. When your Apple Watch only shows normall wear and tear, and you can prove you always used Apples charger with Apple's official cables, Just hold your grounds and demand your legal warranty!
There are however some caveats to this law. First, this law only applies to consumer purchases. B2B is not covered.
Second, you can only claim your legal warranty at the company you made the purchase. If you bought your watch directly from Apple, you should claim it from Apple. However, if you bought it from a reseller, you should claim it from the reseller.