To find out if you're not eligible for ROA you need to pretty much say;
When both parents were born, their Citizenship(s) (Former/Current) how they got the citizenship(s), the visa / residential status of your parents at your birth, when you were born, how often you visit Hong Kong.
I might be forgetting something but essentially there is a checklist of stuff that will help to clarify what status you would most probably be entitled to. I would however like to say that they seem to bend the rules or be lenient in some cases if you were born before 97 as they have a little bit more wiggle room with pre-handover cases.
You will have to fill out the same form anyway, so may as well do it properly. Essentially in my case and other peoples case we filled out a ROA application knowing that it was going to fail but instead provide us with RTL.
My advice would be either post it / put the application in the box -as soon as possible- if you land at 9am, go dump your luggage at the hotel and put the application in that morning. Provide copies of passports, marriage certificates, visas, birth certificates, whatever you've got. Anything they need to ask you for is going to hold up the application.
If you can make it clear cut then it's likely to get processed quickly. I am not sure if it was because I was white and my dad was born in the olden'days that mine was processed and a result mailed within 4 days. It could have been the fact that I (as I have stated in previous threads) provided all of the relevant information and even wrote a letter summarizing the situation including dates and so on and why I should be eligible for RTL.
If you can conclusively prove you're not a Chinese citizen, parents born in HK (* could write a few paragraphs on this post 83 / with regards to hand over, but its not relevant in your case) and that you have been outside HK for 36 months or more then it's pretty cut and dry.
You can probably streamline getting a temporary HKID(totally legal ID, which I have traveled on) in 2-3 weeks if you are lucky and time it right and have your documents in order that make your case clear to determine. If they're busy then well you're out of luck. Getting the permanent ID took at least a few more weeks if I recall.
TLDR; You should get at least RTL & need to provide a lot of specific info/documents, apply in HK, ROP145. Temp HKID 2-4 weeks, proper HKID 6-9 weeks if you're not held up.
Your mileage may vary.