The following paper gives a pretty comprehensive overview of the topic in its fourth section:
IV. Early Use of the Written Accent in Spanish
In general, the accent mark was not employed in manuscripts in Spanish or Spanish
books before 1550. It was only after the Italian
and French models had been created that the
widespread use of accent marks began to be
practiced in Spanish. Between 1475 (the approximate date of the introduction of printing
into Spain) and 1566, the use of accent marks
was negligible. The 1477 Doctrina christiana,
printed in Seville, contained a few grave accents on justícia and fortuíto, but after that
until 1566 the use of accent marks was rare.
After 1566, accents began to come into general use, and by 1570 the grave, the acute,
and the circumflex had appeared, in a sporadic
but increasing frequency, in printed books
(Douglass 1964: 110-12).
In the late 1560s, forty years after the
French and nearly sixty years after the Italians, the Spanish began using accent marks
with increasing frequency and consistency.
The grave accent (`) was the first to be used
beginning in the period 1566-1575. It was used
on the monosyllables è and ò (conjunctions),
à (preposition), and on word-final accented
vowels, as in dexò (note the Greek model).
Late in the sixteenth century, the acute accent
mark temporarily took over the function previously served by the grave on monosyllables,
and was also used on the penults of words
that could be accented either on that syllable
or on the word-final syllable (hállo, ésta).
When the final vowel was accented, either
the grave or the acute would appear.
The acute was especially strong between
1650 and 1676 on word-final syllables and
monosyllables (vió, dexó), and the circumflex
accent, often used in place of the grave in
word-final position (dexô, procurarê), enjoyed
a limited popularity between 1568 and 1690.
The general trend, however, was to use the
grave between 1626 and 1726. A favorite use
of the circumflex was on the word fê to indicate
the previous spelling fee, in the imperfect of
second and third conjugation verbs whose
stems ended in a vowel (traîa, veîa), and on
the regular past past participles of that same
type verb (leîdo, traîdo). Note how this use
coincides with the Greek notion of rising and
Related: ¿Por qué nos quedamos con la forma "sandio" y no con "sandío", tras siglos de coexistencia de ambas?