Omega watches are given a priority over other brands within the Swatch Group (confirmed by the manager). So, category 1 service shouldn't take more than 3 weeks. Mine took a week or so for them to change the seals and do COSC testing a etc.
Based on past experience, I will take this with a pinch of salt.
As for frequency of service, it is basically up to you. Most watches, you should send it in after 5 years, regardless if daily beater or not. To me, it is to give the watch a once over, in terms of getting new gaskets and getting new oil. The oil on your watches will age, regardless of use, so it is good to get them replaced.
I have seen watches go longer, some as long as 20 years without a service. So, should you do the same? Well it is really up to you. Just be prepared to have a long list of parts replaced (likely) and maybe even being told, no more parts as the watch is too old. Yes, having your watch serviced regularly does not solve the problem of parts running out, but at least it is not 20 year old parts since last service.
As for me, my watches (now anyway) goes in once every 5 to 6 years, regardless.