There are multiple factors that limit how frequently the Heal spell can be cast.
Action economy. The number of actions you can take in a certain time span depends on your initiative. This is the first and most obvious restriction on how frequently a mage could cast Heal. With Reckless Spellcasting you could cast two Heal spells per 10 initiative.
Drain. No matter how low the spell level, the minimum drain for any spell is 2 stun. This limits how many times you can cast Heal in the long term. You'll eventually need an hour of rest to recover from drain. From "Step 6: Resist drain" (SR5 Core p. 282) :
the Drain Value is determined using the Force and the listed calculation, but can never be lower than 2. You can resist this drain with your Drain Resistance pool, but every time you cast you risk some drain getting through.
- Heal can only be used once per injury. This limit the frequency Heal can be practically cast, at most once every time someone is injured. Once a character has received magical healing, the remaining physical damage cannot be healed magically. From the Heal spell's description (SR5 Core p.288) :
Any boxes of Physical damage your target still has after any boxes are healed magically can only be healed by time and rest.
- Heal must be sustained to become permanent. The duration code for Heal is P (Permanent) which requires the caster to sustain it for (Force) combat turns before the healing becomes permanent. Sustaining a spell imposes -2 dice pool penalty to all tests as long as the spell is sustained for each sustained spells. While this doesn't impose a hard limit on Heal, it does introduce diminishing returns. It takes a while for a Heal spell to become permanent, and healing becomes more difficult for each Heal spell already sustained.
SR5 Core p. 282-283 :
Permanent (P) spells don't fade or dissipate; their effect becomes a lasting, non-magical characteristic after you sustain the spell for (Force) Combat Turns.
If you sustain a spell [...] you take a -2 dice pool penalty to all tests while it is sustained. You can sustain multiple spells, but the -2 penalty is cumulative for each spell.