Your exposure time will vary from object to object, but generally, the longer the better. With a DSI, you may find that the brighter areas of objects are bloomed out in longer exposures, this is when you will have to do short exposures to get the bright detail and longer ones to get the dark detail and combine them with layering techniques in Photoshop. 10 x5min exposures are more useful than trying to do 1x50min exposure as each frame will fill in the detail and data gaps that are evident in a single exposure. Also, and I think this is the biggest factor, you have to get to know your equipment and how it performs, and this can take a while. DSI's are good entry level units with good software in a single package, learn all you can from these and your scope before you upgrade to better equipment, as it doesnt matter how good the equipment is(within reason, as there is a crap level), if you dont know how to use it, it still wont give you any better results.
