Any slower will seem wobbly, and faster will sound like a quiver in the tone. I suggest that trumpet students practice exercises specifically designed for developing vibrato habits , so physical actions become a part of the individual's subconscious musical reflexes. If done on a daily basis, a student could probably expect a nice vibrato to develop within 3-4 months. The most common vibrato problem I see in young college students is when the tone is held primarily at the upper pitch level, dipping only briefly to the lower level as illustrated in the following example. The second most common problem is when the player's vibrato commences on each individual note rather than being a continual expressive device culminating in the climax of a phrase.
Some students tend to swell louder as well as widening the vibrato on every note.
Both of these problems are caused by the natural musical instincts of students subconsciously trying to match a vibrato they have heard in professional players. For that reason it is very helpful if the student has a teacher giving them private attention to provide a good vibrato model and to be certain they are developing good vibrato habits.