How long downloading takes depends on the quality of your link. But it also varies, because browsers handle PDFs in different ways. It may be better to download a oarticular volume yourself, so that you can then handle the file as you wish, without depending on your browser.
There is no reason why you should not download as many volumes as you have room for. Although much of the material is still in copyright (by the Abbey Trust, unless stated otherwise), all the material of volumes 1-119 has already been published in print. Having your own copy will make research easier.
This site includes only the first 119 volumes. More recent volumes are published on line here. It would be desirable to have a detailed index by author and subject, but it would be very big (or a dynamic site), and we do not have the resources to achieve this: it is a nice dream. And there is no substitute for exploration.