Actually, 65408 isn't the maximum time. It depends on the setting in the scheme.
It's like this:
255 (-1) = 65535 seconds
254 (-2) = 65534 seconds
253 (-3) = 65533 seconds
...
128 (-128) = 65408 seconds
127 (+127) = 127 seconds
126 (+126) = 126 seconds
...
0 (0) = 0 seconds
This is because in a byte value (which ranges from 0 to 255), the numbers 255-128 act as negative if the byte is signed. A two-byte value ranges from 0 to 65535. So imagine 0 as 65536 (unsigned value), from which the said amount of seconds is substracted.
To answer your question, once the timer reaches half of the said value (like 32767), it turns around and goes back to 0.
So, you can customize a precise turn time between 65408 and 65535 seconds using a scheme editor. But this also makes it kind of inconsistent: the SchemeEddy schemes will mostly have 65408 seconds, while the "infinite" turn time set from W:A will be 65535.
Here's an example replay of a 65535 second turn. Press Space and watch.