As a guide, a 4/3mm hooded suit works well for water temperatures down to around 10 degrees, making it a solid choice for early autumn and late spring sessions. From 10 degrees and below, a 5/4mm or 5/4/3mm hooded suit is the more reliable choice for extended time in the water. For the coldest winter conditions, a 6/5/4mm suit is what you reach for when you are serious about staying out.
If you feel the cold quickly, it is also worth pairing your suit with wetsuit boots and neoprene gloves to keep your extremities protected throughout your session.