When God spoke to Moses out of the burning bush, He described Himself as "I am". He said He simply is.
We think of eternity in terms of "time" as that is our experience, and so we imagine a very long time. To be honest though I sometimes wonder if "time" actually exists. As far as I know there is no physical evidence for it in the same way as for the quantum particles for example, although even they can be described as standing waves if my limited understanding is correct.
If time doesn't exist, then we are also part of God's infinite plan in that we just "are", but only due to God's creative action.
There's one opinion on time maybe not existing here -
Cutting-edge theories of physics suggest time may not be real – but even if they’re right, life can still go on as usual.
theconversation.com
Trying to pin God's eternity down is like to trying pin Him down personally - He occupies the entire universe and beyond; maintains the universe; watches, listens and monitors the thoughts of 8 billion people simultaneously, remmbers it all and plays it back to us when we front up to the judgement seat; watches over the more than 100 billion souls who have already died and are either in heave, hell or purgatory; judges 150,000 new fatalities every day (about 100 a minute); maintains an unbreakable barrier between heaven and hell; communicates with poeple (sometimes) and the spiritual beings; while all the while hiding Himself.
That's why I quote Isaiah at the bottom of my posts.