PHB 148 on silvered weapons:
You can silver a single weapon or ten pieces of ammunition for 100gp. This cost represents not only the price of the silver, but the time and expertise needed to add silver to the weapon without making it less effective.
The rules are not specific as to how much silver is required, or rather, how much of that 100gp is in silver components. With no particular reference, I can only assume it's half the cost (50gp worth of silver). This seems to be supported by the following:
PHB 187 on crafting:
For every day of downtime you spend crafting, you can craft one or more items with a total market value not exceeding 5gp, and you must expend raw materials worth half the total market value.
If raw materials is half of the 100gp value of silvering, does this mean that it would take 20 days to augment a weapon with 50gp worth of silver? Furthermore, would it also take 20 days if the PCs offered 100gp for a professional craftsman to do the same?