Hi Jman,
Phil hart's review is a pretty good place to start.
http://philhart.com/content/star-adventurer-review
There are multiple factors that limit the maximum exposure you can make on any equatorial mount.
1. Polar alignment accuracy
Frank bennett's article on polar alignment -
http://celestialwonders.com/articles...ent/index.html
and his drift calculator
http://celestialwonders.com/tools/driftRateCalc.html
for example using the calculator I entered a polar alignment error of 2 degrees, for a 3 minute exposure the stars will drift 95 arc sec at the celestial equator and 70 arc sec at the declination of Eta Carina and the Cross. This is suitable for a 3 minute exposure with a 10-15mm lens to keep the drift to less than 1 pixel in the camera.
Example 2
Let's say you can get your polar alignment to <±1 deg
taking 2 minute exposures
and you'll accept 4 pixels of drift seen as 1 pixel when the image is viewed screen size ~ 25% of full res.
Then you're looking at a drift of around 30" at the celestial equator and 24" at the Cross. You can then use a 135mm focal length for a 2 min exposure. Exactly what Phil found.
2. Periodic Error
On the SW star adventurer this is about 50" however if you keep the exposures short, you will only see part of this drift during an exposure. You may need to take several, toss those with big drifts and keep those with smaller drifts.
Sorry there is no one line simple answer to your problem
Joe