I always only offer water (once bubs can have water) so around 6-8mths in a Mag Mag straw cup, they eventually they get the hang of using a straw & not needing to tip the cup up to drink out of, so Maddy (bottle fed) was using a straw for water & milk from about 8months onwards by 10 mths she refused her bottles & only used a straw in a cup (I find Mag Mag the best they are about $12ish though, but Indah is now using Maddy's old one (8+yrs later)
Indah was breastfed but from 6mths onwards had Expressed milk from a bottle as i was back at work, I also introduced the Mag Mag cup (pigeon brand) around 6ish months when she started solids & she has been drinking Milk & water from these cups from about 10months (although she was still breastfeeding 2 times a day til about 14mths) she'd have cows milk in a cup during the day or water! (I now have 3 Mag Mag cups & rotate them)
I have never gone on to a sippy cup as its' just an added step to try to teach them & then wean them off as such, Indah also can use an open topped cup (no straw by herself, but usually we have some spillage!)