As long as the website is public available there is no problem using google maps in your own map application.
If the site requires login or is only available internal in a company, then you need to purchase a license for it.
I do not think there is any limit to the amount of requests to the google map server or anything.
Thanks for your reply. Our application runs in an intranet, and we are experiencing some delay and partially rendered google maps layer (it did not ocurred during development). So we think it must some limitation imposed by google. Our application has now about 10 users.