You could try to use a combination of bytes/second and 
connections.  If you set a low bytes per second, say 500 
bytes/sec that'll make each page take a lot longer to 
download, and with only one connection that 500 bytes/sec 
will be going to a single page only (single request).  If 
you estimate each html page to be about 10k, that's 20 
seconds per page, or 3 pages per minute (essentially 3 
requests per minute).

You can therefore control requests per minute as a function 
of the average page size, your bandwidth allocation 
(bytes/sec), and connections.  Hold the connections to 1, 
and adjust the bytes/sec value as necessary if you observe 
the pages being significantly smaller/larger than 10k per 
page.

Alternatively you could try to use a Windows macroing 
program (maybe the free AutoIt would work) to periodically 
Pause/Resume Winhttrack.