If you're sat right on top of the router (so close that you may as well plug a network cable in),
and there aren't any other networks in range,
and you're the any one using the network,
etc. etc.
An AC1900 router is going to be 600Mbps on 2.4GHz and 1300Mbps on 5GHz.
You can't be using both at the same time so you're down to 1300Mbps.
As a rule of thumb, under perfect conditions, you can half the rated speed; so 650Mbps or 80MBps (ish).
It should be fast enough for your planned 250Mbps connection.
Don't forget that Gigabit is full duplex (2000Mbps) not half duplex and that every connection has that full bandwidth available all of the time.