So, what is lead glass? let's look up what it is...
Lead glass, commonly called "crystal", is a variety of glass in which lead replaces the calcium content of a typical potash glass. (wiki)
Is it realistic?
Now, this helps us to know what is mixed when making lead glass, but the common glass before 1700 AD north of the alps was forest glass. This Glass used Potash in mixture with Calciumoxide, often tinted green by iron. So much for lead glass being common. It is by far not unheard of, but not the most common type.
But we want lead glass!
But let's assume someone really wanted to make lead glass for they wanted to sculpt it into fancy shapes or because they are paranoid... What would we have in terms of lead content? Let's assume we take some common (more or less contemporary) glass types and just replace the calcium with lead. Let's look it up.
[O]rdinary glazing and container glass is formed from a specific type called soda-lime glass [...] (wiki)
Container glass: 10.5 % CaO
Flat glass: 9 % CaO (wiki)
Lime, CaO: 4/95 (15th century BC) 8/93 (1st century BC) 12/92 (13th century AD) 10/97 (14th century AD) 10/195 (modern) (wiki)
So, about 5% calcium content as a bare minimum for historical glasses, with something around 10% for more 'contemporary' medieval glasses and up to 13%. Just replacing that with lead allows us to estimate a 10% regular content. But there is also more!
Crystal can consist of up to 35% lead, at which point it has the most sparkle. (wiki)
So, how thick?!
Taking this into account, then a lead window would need to be between 3 to 20 times as thick as a lead sheet to block detect magic. "Thin lead sheet", however, is undefined, so I will assume 0.5 mm or less for a lead sheet, which is about the thickness used to line roofs.
Common window planes nowadays are 3.175mm (1/8th inch), but we'd need slightly thicker windows of 5mm to keep people from prying with an average 10% lead salts - which is by far not out of question for medieval stained glass windows.
With 5% lead content, we go towards the rare - but not unheard of - 10 mm windows.
For really high lead content (35%), 1.5 mm - which is about as much as watching through common glasses nowadays - would be enough.