ermmmm...... why? How do you want to limit download times? You mean have your course accessible only for limited times through a day or a week? Like only available from 7:00am to 7:00pm, five days a week? That is the only way you can limit download times that I am aware of. One of the fundamental principals of Moodle was to lead the way in anywhere/anytime learning, so no consideration of placing time limits on accessibility were really made. You can make courses available or not, but that's about the only limitation.
If this is what you want to do, then set a cron job to run placing the site into Maintenance Mode at a time and another cron job to reopen the site at the required time. I suggest this will destroy your credibility as a learning provider, but that will be your problem.
If this idea is being forced upon you by your ISP or host, then look for another ISP and/or host. There are plenty of providers willing to assist you in any way you require, The best part, you don't need to even be in the same country as your host, pick a host from anywhere in the world, they will even set you up with a webident, if you need one.
You, however, have given so little information about you, your site, your circumstances, all this is just hypothetical.