Quite huge given the good spec.
But it is like asking how much time you need to join a city given the max speed of your car.
The road you take, speed limit, traffic, time you need to sleep, time you take to re-fuel and/or eat could or would be as significant in the end.
Depending on the efficency of your designs, use of mods, use of commands , biter settings, the megabase can vary in size at least from 1 to 10 before your UPS are impacted.
The number of bot it can handle depends on their activity, a million sleeping bot doesn't require much CPU power
, also, if you have a megabase and did research to bot cargo size to the max and are now researching higher an higher bot speed ( which is infinite ), you need fewer and fewer robots because each of them is faster and faster.
One common metric for megabase is "science per minute" (SPM), how much a base can produce, more than the "size" itself, as would be represented by an area on the map or the number of machines.
You can read on the forum many posts about other player megabase ( with XXX amount of SPM ), some of them describe the hardware they used as well as the design of their factory, it would give an idea of the conditions you'd encounter if you use a similar architecture for your factory, given a certain hardware difference.
Also one other thing, you mention "lag", but one could be more precise, some players don't mind reducing the game speed, instead of having UPS jumping between 30 and 60 you drop 50% game speed and you don't get "lag" effect, but a rather smooth and less frustrating general slow-down. This can be helpful to "finish" a base if you start experiencing difficulties on a planned thing.
Then it's up to you to make a base, given your dedication to efficiency, time spent to learn and improve things and so on, you'd get a different answer
