Personally I make an active check every Tuesday (It is actually in
my diary every Tuesday first thing) and update my prices if needed.
I also keep an eye on supplier invoices that arrive throughout the
week and if I notice a substantial change, then I will also change my
prices there and then.
All my prices are worked out dynamically on my website so I only
have to change the master price of silver/gold/platinum once and it
has an impact on all the prices thoughout my website.
Some sites ( I would imagine the majority ) are written in old
fashion html which means that each item on each page would need
re-writing individually. I can see for a business that has a website
with 500 products that this could be quite a problem. Even more so if
you are not computer literate and require a web designer to make the
changes for you.
The only way around this is to have your product prices on a
database or to price dynamically like I do.