How long will this alum frame last?
A crack developing there, is most likely too much seat post out of the frame, or in actual fact, not enough seat post being in the frame.

The biggest problem with a crack in this location, is that it then causes most force to be placed on the join between the top tube and seat tube. You also have to think about what would happen if the frame were to suffer a critical failure in this position - it will most likely be landing from a drop or, more likely, while sitting and hitting a hole/bump; neither great options and the more likely of the two being much more painful!

Drilling a crack is to minimise crack propagation. And there are few repair options with this style of frame (for sensible money). 

This can be welded, but you would risk this just getting worse. If I remember correctly 7005 using artificial ageing rather than a traditional heat treatment - a filler material could be used between the post and frame to bond them and strengthen the seat post. The sensible money "option" if you really want to keep the frame is to get a very long seat post (ensure that there is a lot in the frame, around 20cm+) and keep checking for further crack propagation. The best thing to do here would be to contact a specialist aluminium frame builder and ask their opinion.

To answer the question in the title - aluminium work hardens quicker than other materials, and tends to crack due to fatigue much quicker than say carbon, steel or titanium.

Unfortunately I think it could be time to consider a new frame.