+1 Martin.
You can use rules of thumb or refer to the various tables advice etc., published on-line by some very accomplished APs.
From experience - unguided - Canon 1000D / 450D - ISO800 3 minutes is a good place to start or ~2 mins at ISO1600, then adjust for your sky conditions. It wont be the same for everyone - just a guide.
But please do DITHER with older DSLRs. DITHERING will cover so many unavoidable flaws. It's very easy to do even manually with a hand controller. APT and BEOS have the capability built in.
Did I mention DITHERING???
Dithering... dithering... di..th..er..in..g...