Discos are amazingly stable. You'll feel like you're going to role long before you're actually in danger of rolling. How far you can lean depends on many factors,,,your suspension mods, weight distribution of gear & bolt-ons, whether or not you have sway bars (sway bars work the opposite off-road,,they increase body roll as they prevent full articulation), etc
Hit some trails with your local Rover club & some experienced drivers,,,it's always more comfortable to experiment when you are with a group. You'll soon have a good feel for how far you can lean & it will surprise the hell out of you.