I got a foot measure thing from amazon to save having to keep going into stores, and then have ordered a few from shein. Shoezone have a good deal on atm with 2 for £10 as well.
My local kids shoe shop said every 12 weeks to me. I got his first couple of pairs from there but at £42 and £60 I don't want to keep spending that every 3 months. I bought a foot measure from amazon to check his foot size... however... my local kids shoe shop also said that toddler shoe sizes aren't standardised so what size they wear in one make, they wouldn't necessarily wear in another make. I saw a hack the other day where you cut a length of straw the size of your LOs foot and if in the shop, pop it in to make sure it fits, you want about 1cm of wiggle room x
We go every 6/8 weeks with Clark’s and I just use the excuse that I brought his new size in the sale so I don’t feel so guilty. I then go on the outlet store or Vinted to buy them also. £42 his first couple of new shoes were, I don’t even spend that on my own shoes!