From around 18 months of age, a toddler's day sleep needs can really vary. They can be on one nap at this age, it may be anything from 45 minutes - 2 hours, still having 11 - 12 hours sleep at night.
If we are aiming for a 7am - 7pm routine, having the nap from 1230 - 2.30pm would be ideal.
The biggest reason to reduce the length of the nap is if there is a delayed onset of sleep in the evening. If your toddler is resisting settling to sleep / taking a long time to settle, I'd first try waking them 15 minutes earlier from their nap, if this doesn't help, then try trimming a further 15 minutes off. If night waking or early morning wake ups start to become an issue, this can also be a reason to trim the day sleep.
Most toddler's drop the day nap completely between 2.5 - 3 years old. If you're really lucky, your toddler may still happily have a short nap during the day without it affecting night sleep closer to 3.5 years of age!