I'll tell you what I know about a really long-term relationship: my parents, who have been married for 60 years.
Do you have plenty to talk about if you spent time alone together?
They still have plenty to talk about. They've always enjoyed reminiscing, even when we were kids. They talk about their dc, their dgc, their cousins, their friends, domestic issues, current affairs, almost anything and everything is grist to their mill. Yet they are also, and always have been, comfortable being together without talking.
How would they act if you’d had a tough/tiring day (eg at work)?
From their adult child's perspective, this has been the main friction point in their relationship. At the end of a difficult day or week dm would want to unburden herself whereas ddad would want some time alone to unwind. Ddad is really bad at reading the signs that dm is emotionally drained, though he notices when she is physically at the end of her tether and looks after her very carefully but in a very bossy way. He is a problem solver rather than a listener, and gets frustrated when his advice isn't taken or the other person doesn't change the situation that is causing them stress. This applied to his dc as well as dm. Once ddad understood that dm was stressed out and needed a break, he would bring her coffee, order a takeaway for supper, send us to play outside if we were being noisy.
Do you find yourself craving time apart from them?
No. They are comfortable being in separate parts of the house, but like knowing where the other is and often just wander by to reconnect. Ddad will say if dm is distracting him, but dm rarely feels distracted by him. When appart outside the house they regularly check in with each other. When ddad worked abroad (pre-mobiles and cheap calls) he would phone almost every day and dm would write to him almost every day.
Do you find your spare time is filled with things you do together, or separately, or a nice mix of both?
A nice mix of both.
Do you worry about your relationship ending suddenly? (Eg if you argue, or do you worry about being cheated on)
My parents argued, but always made up. Dm felt secure in ddad's love, describes him as her rock. I don't remember atmosphere between them ever lasting long, or having to tiptoe around them because they were cross with each other. Ddad considers himself an honourable man with a standard of behaviour to uphold. And he always meets that standard - there's no hypocrisy. He is appalled by dishonesty. AFAIK dm trusts him implicitly and always has, even when he worked abroad.
Obviously what a child knows about her parents' relationship is incomplete. But dm and I talked a lot, especially as an adult when I was struggling to find a partner, and also when ddad retired and dm was finding it a challenge to readjust.
I know, I married a man with many similarities to my ddad. Not that I realised it at the time!