It varies from character & also by their location, but if you show up at the start of the meet time listed in the app you can expect at least a 20-30 min. wait for popular/rare characters. A lot of people will stake out the area 10-20 min. (or more) before the posted time so by the time the meet time comes the line is already forming. For this reason it’s not uncommon to see just 5 min. windows posted for their meets & that usually means that in that 5 min. they’ll allow guests to queue & then cut the line off and possibly even earlier if by the handler’s estimate the character will not be able to make it through the queue in the amount of the time is actually scheduled for the character to meet (which isn’t posted on the app but is known to the handlers who are responsible to manage the character wrapping up on time).
For the character meets that have larger blocks of time and/or established meets the above won’t apply, but generally if you go earlier in the day the wait will be less. We’ve noticed though that for Pixie Hollow Tinkerbell is always advertised as being there 8am-3pm but often isn’t there until 9 or sometimes later, but hers is definitely one to go as soon as she is open bc her line gets long & once it warms up is in full sun & moves very slowly so isn’t an ideal wait.
Royal Hall will often have only 1-2 princesses right when it opens so we like to wait until 9-9:30 (or 1 hr-1.5 hrs after it opens) to get the all 3 princesses. The wait might be longer (20-30 min) as opposed to 5-10 min in the first hour, but you’ll meet 3 princesses so it’s a good trade-off. Also most days there is often a lull around lunchtime so noon is a great time to get in line & have less than 20-30 min. to meet 3 princesses.
And then if your child will be into Pirate characters at all it’s worth noting that for Jack Sparrow & Redd (who are both fantastic to meet) they will often do a very unusual meet style where if their line is starting to build up they will start walking the line to a different location in an attempt to throw people off. They’ll do it really cleverly saying their “just recruitin’” or going on an adventure of some sort, but most of the times we’ve done these meets, the guests around us & have all been agreeable about following in a line formation to wherever they go next & the character keeps interacting with the guest who is at the very front making their turn a little extra special, but it does often discourage more from getting in line or cause those who are towards the back who can’t really tell what’s going to bail so it’s effective in keeping their lines manageable especially where they meet in more cramped quarters (the walkways of New Orleans Square are some of the narrowest in DL).
And since I’ve brought up the unique situation with Pirates characters, there are a lot of characters (mostly in Fantasyland but can happen in Town Square or random places in DCA) who will be roaming only which means they are happy to interact with guests/sign autographs as they walk along between destinations & will “pause” if you have someone ready to pose a photo (but not guaranteed) but they will not stop to have a formal queue form & it will a free-for-all requiring you to be a little more bold than you would otherwise be in a traditional line up to meet. There can be times though when no one else is taking advantage of or noticing a roaming character where you can just walk up to them & end up with a lovely & lengthy one on one interaction so again, a very big pro that counteracts the cons (might be a scramble if a lot of guests are trying to interact with the one character & no formal line; hard to notice since they can tend to blend in and move very quickly among the crowds so you have to be obervant/in the right spot at the right time).