I will say wait until the baby shower if in case she ishhaving one to see what you get n what you need to buy specially cuz people gives a lot of clothing n many times the baby doesn't even uses all of it.
I agree with the first comment its all up to u, I personally don't like to buy anything until the last few weeks of my pregnancy. I was a personal experience and also it allows u to see what u may need. Ex: baby showers normally take place in the last month of pregnancy, people may get u all those things u need and more, so u would only have to purchase what u weren't gifted.
Any time you are ready. Lots of people wait til at least second trimester in case anything happens. Many people will also wait til their 18-20 week ultrasound so they can find out the sex of the baby. That way they know if they are purchasing pink or blue items. I was shocked at how few gender neutral items there really are out there anymore. I personally waited til I found out what sex the baby was. There are no rules though- some people find it fits into their budget to buy a little bit at a time every payday. I knew some ladies who purchased diapers that way.