Mining grins will become possible as soon as the “mainnet” is released, which requires a fully developed, well-tested and hopefully audited codebase. How long will that take? Software timelines are notoriously difficult to predict, and this is particularly true of open source projects that rely on volunteers’ time. So the best answer we can give you is “as long as needed”. A less good answer would be “at least 9 months from now” which would be a guesstimate that’s likely to be wrong.
As far as buying, it’s completely dependent on exchanges and so outside of our control. In any case, don’t try to buy anything until after we’ve announced the mainnet release.