Yes, because it's a Cantrip; they don't spend slots so are unlimited w/ a few exceptions which are explicit. Plus "at will" also means unlimited.
Note that you must still expend the action cost.
I've had a few players over the years equate at-will with "free action," which isn't the case.
Cantrips and focus spells are nice because their spell rank usually scales with your class level. So if you're a 7th-level ranger, for example, your cantrip(s)/focus spells will be 3rd rank.
The calculation to determine spell rank for cantrips is simply half your level, round up.
Spells that are not cantrips or focus spells--generally those from your spell slots, scrolls, or staves--generally do not rank up in this fashion.
A 3rd-rank lightning bolt will always be 3rd-rank if cast from a 3rd-level slot. It will remain 3rd-rank even if cast from a 4th- or 5th-rank slot, unless you're a prepared caster such as a cleric, druid, witch, or wizard.
I hope that helps!