Hoyas are fairly easy to grow, but the most difficult thing to master is to get watering down. Over water and you get root rot. Under water and you'll lose new growth and have roots dry out.
Hoyas need varying amounts of water depending on potting mix, pot size and climate they're grown in. Rule of thumbs are difficult to follow as the growing conditions vary so much among different types of hoyas.
If the mixture is very airy, you can even water your hoyas daily.
The rule of thumb I use in a low humidity environment in Southern California is to water around once a week and adjust based on personal observation. For plants that are actively growing, I would adjust and water more often. If the pots are larger, I'll adjust and water less often.
I also water less during the colder months and more during hotter months.
One of the biggest risk of over-watering is root rot. I've lost my share of hoyas to root rot, but learn that you can usually catch the rot before it is too late to re-root the plants. Look out for hoyas that have suddenly have leaves that turn yellow. When that happens, I'll check the roots to make sure it isn't rot. If it roots have rotted, I will reroot in sphagnum or perlite.
Hoyas grown in higher humidity conditions won't need to get watered often. I usually water them every 2 weeks in the winter.
In summary, watering hoyas is tricky. Sticking to a schedule doesn't always work, but the best way to know is by being aware and observing your hoyas often.