I have a standing every 2 years on each of my 2 10k gallon tanks. Each year, one gets pumped. That means they both get checked regularly.

Even if it costs $200 for the appointment, it's worth it to know. They can also likely tell you the size and what they suggest for your situation. They should be able to schedule it for you so you don't need to remember.

FWIW, ask around to find a reliable, honest septic service provider. Ask your neighbors. Ask your plumber. Ask at the hardware store (if you have an independent or franchise, not a big box. Ask at the plumbing supply store.

we have ours pumped every 2-3 years, house of 4. Rain wont fill it up, because it will eventually end up in the leach field and levels will go down. Solids are the risk when dealing with overflowing septic because it will get into the pipe to the leach field and clog that, causing a backup in your plumbing.

as for the smell, it depends, every house I have lived in with septic would get a wiff from time to time depending literally on the direction of the wind. The house that had the least smell was a 3 story house in the north east where its cooler, the one with the most was a house in the south east that was only 1 story.

Height of the vent makes a difference, as can using products like Rid-X to help keep things broken down particularly fats which can really stink up a septic system.

Find out if your local county or municipality has a required timeframe. By us it’s every 3 years.

As far as how to tell in the short term, is there a tank between your house and the drain field that you can access (has maybe a round metal or concrete cover or a plastic riser with lid)? If so, lift the lid and see if you can see the top of the pipe going in the direction of the drain field. If so it’s likely draining fine. If the liquid is near the top of the tank and you can’t see a pipe, you’ll want to get someone to look at it sooner rather than later.