OP it depends on the size of your baby! My LG finally fit into 0-3 clothes at 11 weeks (we first wore tiny baby, than 0-1mth). I bought a 0-1month snowsuit in Tesco that fit her (massively big in the beginning) from when she was born in December through April (when she no longer needed it). She can still fit her 6-9month sleepsuits and clothes at 11.5 months, although is comfier in 9-12 (but only in the last two weeks). She wasn’t born tiny (+7lbs) and likes her food but is narrow. Other babies born at the same time have been in 12-18 months for ages! And some in 18-24.
My tip to myself in hindsight would be not to buy too much. As clothing for babies is fairly seasonal (May in the Uk was suddenly warm. Long sleeved wool suits weren’t going to work! No matter how cute they were).