Only about 5 days. I just got back from returning it through FedEx. I'd urge you to return yours as well if possible, the Samsung rep on Facebook I spoke to said they would forward this issue to the dev team, but you know how that goes.
Sadly, no. It was a temporary fix. For a while, I was getting my notifications instantly. Then (even though I didn't reboot) it started taking 10 minutes to get them. Then 20 minutes, then half a hour. Something is wrong with this phone, I don't know if it's a bug or what but it's borderline unusable in this state.
Had my S9 a week now and it behaves just like my S7 did with delayed notifications. I have to run the ADB command as stated in the begining of this thread. Even then it does not match my Ipad in response time to receive notifications. I knew this going into the S9 purchase but there are no other Android phones that met my width requirement for a new phone purchase. Hope to hear positive news from your meeting!
I had the same issue on my Samsung S9+ and it appeared that the cause was that, when I did restore from my old phone Nexus6, Greenify app was restored too and cause the doze issue to only affected app WhatsApp.
After removing it, even phone was on idle mode, I can still receive and send notifications from WhatsApp Web side.
Hope this can help too.
Dropped my phone off today at Samsung.
Told me they have never heard of the delayed notifications before.
I showed them this thread they really did not care.
They said they will factory reset my phone etc.
Picking up tomorrow.
Will report back when picked up.