In chapter four of Wheelock's Latin, it states that:
In spelling the genitive singular of neuter (and masculine) nouns with a base ending in -i- the Romans sometimes dropped that vowel, e.g., cōnsilī for cōnsiliī (see above) and fīlī for fīliī; in this book, however, the stem vowel is regularly retained.
How often is often? Did this happen only with select words of this declension or was it pervasive? Is this something that became more common over time as Latin evolved or was it distinct to a specific period or location?