It's done when the client receives the last query result packet from the server.
If the client is slow consuming the results then this will delay the overall query execution and it will be suspended waiting on ASYNC_NETWORK_IO.
The query doesn't run to completion and get the results buffered somewhere and only then send them. So certainly seeing some results after 3-4 seconds is no indication that the query is "Done".
e.g. I ran the below in SSMS with both "Live Query Statistics" and "Actual Execution Plan" enabled.
DECLARE @X VARCHAR(MAX) = REPLICATE('X',8000);
SET @X = REPLICATE(@X,125)
SELECT TOP (20000) @X
FROM master.dbo.spt_values v1, master.dbo.spt_values v2
OPTION (RECOMPILE) /Not sure why but SSMS doesn't show the live plan for me in this case without this/
Results started appearing in the grid more or less straight away.
When it completed the Query Time Stats in the actual execution plan showed an elapsed time of 80,668 ms with the wait stats in the same plan showing 60,184 ms of ASYNC_NETWORK_IO waits
Viewing the live execution plan whilst it was running shows that rows are still flowing from the execution plan operators right up until the end (e.g. below is how it looked after a minute for me by which time it had returned 16,393 rows)
If the above query is running at an isolation level requiring it to hold locks until the end of the statement then this will mean that locks are held open longer due to the slow client.
The above query has no "blocking" operators so in the above case rows start streaming to the client more or less straight away.
For more about this you can see a related but inverse question about "When do the query results start"
