Going to onsen with partner

Silly question but... if you go to an onsen with your partner and you each go to the separate sex areas, how do you arrange to meet outside again at the same time?

Do most onsens have clocks inside, or can you take a little cheap digital watch? How do you ensure you don't spend ages in there while your partner sits outside waiting and getting bored?

Do most onsens have clocks inside

Only a minority of baths have a clock inside the bathing area. But probably a majority of baths have a clock in the changing room.

or can you take a little cheap digital watch?

You are free to bring your watch. But don't put watches into acidic water or they may be damaged. Some onsen water has a pH close to 1.0.

How do you ensure you don't spend ages in there while your partner sits outside waiting and getting bored?

My wife and I usually say that we meet again in X minutes. Both of us have a good inner clock and we usually get out around the same time. Also, many onsen - although not all - have a nice waiting area with drinks, maybe a massage chair, a shop or tatami room, etc., which makes it not unpleasant for one of the two to do some waiting.

If one of you wants a long soak, and the other one just wants a quick dip, then the quick dipper takes the room key and you meet back at the room.

Oh, that's the most romantic part! There used to be a song about a girl waiting too long that her wet hair got so cold.

On the more practical side, most if not all onsen facilities have a resting area where you can meet. There, you can enjoy free tea and water and lie down on the tatami, or often you can even buy beer, snacks, meals and even put coins in the massage chair.

To answer your question, usually there is a clock on the wall of the dressing room, or at least you can keep your watch there. For ladies who dry their hair or put cosmetics on, this still can be handy.

Meanwhile, ideally you should discuss beforehand to make sure your partner wants to enjoy a long dip in the tub, wash hair or just wants to shower and get out. These are typical checking points for couples and families bathing separately. Humans do just about the same things in a bathing room, so all you need to know is the "extra" things.

So, not many people decide the clock time to meet. They just say things like, "I want to wash my hair, so I'm going to take long." "Okay then I might be taking a walk, but we can still meet at that resting area." Nowadays you can also say "Text me when you're done."

Thanks for your replies all. It sounds like onsens are a good place to relax whether you're in the bath or out of it, so I won't have to worry!

For public onsen, it separate sex and it do not mix.
If you want to bath with your partner then book a private onsen.

For public onsen, it separate sex and it do not mix.

Unless it's a mixed public bath (konyoku). In that case you could enjoy the bath together.