You can certainly bow any time you want. It's traditional that whenever there's a Buddha statue in the room that we bow to it when we enter and when we leave, so it's a tradition. Although it's traditional to bow when the statue is there, it's not just the physical action as bowing has a deeper purpose. Bowing is linked to a reflection on the Buddha, the Dhamma and the Sangha.
Regarding the Buddha, he discovered the Truth and gave us the Dhamma and a path to discover this truth. Can we really feel deep gratitude and thankfulness to the Buddha? That's what we're bowing for, it's the recognition that this person was greater than me. This person was a wonderful human being, and we're humbling ourselves at the same time.
When I was growing up in America, as a kid in elementary school, we were taught to bow, and the girls were taught to curtsy. We were taught to take our hats off in the winter whenever you passed by any woman. We were taught this form of physical respect. In Thailand traditionally when you say hello to someone, you "wai" them, putting your hands together and going like this towards your chest if it's a friend. If it's your teacher or someone you respect greatly, you put your hands up to your head. Now that's just a short form of getting on the floor and bowing.
So with the bowing we are trying to develop this respect and a humble feeling inside which is lacking greatly in this modern fast-speed world, and realize we are not the center of the universe or the greatest person that has lived on the planet.
We try to develop a real feeling of thankfulness towards the Buddha for his teachings and the other teachers for over the past two and a half thousand years who preserved the teachings. So indeed the bowing is much more than the physical form and you can do it anytime whether there's a statue or not.
One thing that some people do when they're feeling a bit down due loneliness and there's no Buddha statue, is to imagine a Buddha statue in the corner and do the bowing, the physical action in their mind.
This can be greatly inspiring, it can bring you out of that loneliness because you feel connected again. This is why we do teach that when you're back in your normal life, to have a short form of the reflections, the Triple Gem, and a little bit of Compassion/Lovingkindness before every formal session that you do, to do a little bit each time. Whether there's a Buddha statue in the room or not, reflecting on the Buddha, the Dhamma and the Sangha, and raising those feelings of respect and thankfulness can greatly ease that feeling of loneliness.