I'd suggest starting with something around 100g carbs a day, and then adjust it (down) according to what you need to get or maintain your weight and/or glucose levels where you want them. No one size fits all, so it's a how long is a piece of string type question! So start at a highish level and see how you manage - it needs to be a sustainable and healthy diet, so there's no point in going ultra low if you can't handle it and end up falling off the wagon. Slow and steady is best.
Have a look at one of the low carb nutrient calculators to help you - see
http://www.phlaunt.com/diabetes/DietMakeupCalc.php or ask Google about others.
I've been down as low as 20g carbs a day short term to get into ketosis, but am happier with between 30 and 50 g a day. It keeps my glucose levels stable and I can eat many of the foods I enjoy too.
Robbity