In my shul growing up, there was a simple rule. The seat is "yours" till ashrei. If you aren't there by then, then anyone can sit there.
Also some shuls appoint someone to be the "welcomer" to guide people to where there may be an empty seat.
As a guest, it's really off-putting when you are really deep into a tefila and someone taps you on the shoulder - "my seat". (I understand doing it for an older child, who needs to feel like he belongs and has a place in shul... but stam a random mispalel?)
+3
When I was growing up, the seat was reserved for you until Borchu. Where I daven now, there is no official rule, but there should be. I believe that paying for membership and getting a seat doesn't mean that the seat belongs to you fully (Can I take the chair home after davening? Why not, I paid for it!), rather it means that you have first dibs on the use of the seat. You should not be able to force it to remain empty when you are not in shul.
In my shul, there are often multiple minyanim. Do I own the seat for all minyanim, and I can make sure it's empty for all of them, even though I'm only using it by one? I think not. I think that it just gives you the right to use that seat when you are there. And if you are going to miss half of davening, you should not be allowed to prevent someone else from using your seat for that tefillah.
When I'm a guest in shul and I'm not sure which seat is available, I take a seat and I keep switching to other open seats every few minutes, so in case I'm in someone's seat, they can reclaim it. But after Borchu, I stay put, assuming that it's really vacant.
