Automated queries and query limits

Thanks for providing such an exceptional service. 

I am carrying out a small scale web crawl (for an MSc research 
project) and was looking to attempt to validate those pages found 
that have a doctype (currently this stands at around 10000 pages) 

I was wanting to check if there was any limit imposed on the 
number of queries made per day to the validator and if automated 
querying (albeit slowly) was frowned upon.

Thanks for an information you can provide,

Andrew.