Distances and Speeds across the franchise
A while ago I participated in a discussion and the distance between galaxies - and the subsequent travel time came up. Here are some numbers to put things into perspective:
Distance between the Milky Way and Pegasus: ~3 million lightyears
A 304 takes about three weeks - let's say 21 days - to cross that distance. That's a travel speed of 52 million lightyears / year - or 142.000 lightyears a day. So at best speed, a 304 can cross the entire Milky Way within just one day.
Now, with a ZPM, that time is reduced to 4 days - that's 750.000 lightyears per day. At that speed, you need about 3 hours to cross the Milky Way - if Asgard Hyperdrives work at that efficiency, no wonder they don't use Stargates.
It gets interesting if we now think about other intergalactic distances:
The observable universe is about 93 billion lightyears in diameter. With the speed of 750,000 lightyears/day, it would take 340 years to cross that distance. We also know that Destiny is "several billion lightyears" away from Earth - at a distance of 10 billion lightyears (that's the upper limit of what I would call "several") - a ZPM'd 304 would take about 37 years to cross that distance - add a few stasis chambers, and you're smiling. If we use a lower number, a rescue mission could've reached Destiny in the time since SGU went off the air.
The interesting part is the Ori galaxy: The Ori, with their near infinite knowledge probably had the means to give their followers fast hyperdrives - but they still chose to create a Supergate, disregarding the issue that one small entrance could - and did - become a bottleneck. So the Ori galaxy is too far away, even with the best hyperdrives imaginable (and I think a ZPM'd 304 is as good as it gets), to support an intergalactic invasion. However, the Alterrans used a ship to cross that distance - and Ancient Hyperdrives were not as effective back then as they were during Lantean times, and even then it seems that the Aurora-class intergalactic hyperdrive was just capable of keeping pace with a regular 304 (the Orion was roughly as fast as the Deadlus in SGA).
So, with that in mind, I'd say the Ancients didn't travel for more then 50-60 years. At the speed of a regular 304 that would result in a distance of 3 billion lightyears. A ZPM'd 304 would cover that distance in 11 years. For the Ori, waiting for 11 years to send reinforcements after the Supergate was blocked in SG1 S10, would probably be too much.
Now, the last part is mostly speculation. The speed of a 304 with a ZPM is still impressive - and I wonder, with the hyperdrive capabilities the Asgard had and the longevity of their species (due to cloning), why they didn't venture out far into the universe to explore?