"what factors could one use to justify limited language drift"
There's no one answer to the question of how long it takes for language to drift. There is no predictable rate of so much language change per century - indeed it is difficult to know how the amount of language change can be measured. It depends on the culture and physical situation of the people concerned.
Physical isolation, such as being spoken on an island, tends to prevent a language from changing by the obvious means of preventing other, potentially "corrupting", languages from ever being heard. Icelandic is an example. The medieval Icelandic sagas can be understood by modern speakers of Icelandic.
Literacy tends to preserve language, but the tendency is not absolute. For instance Classical Chinese is perhaps the language that has been able to be read by most generations (although Tamil may be a competitor). However because written Chinese gives scant guidance on pronunciation, that has probably changed a good deal even though the grammar and form of the characters remained relatively stable.
On the other hand lack of literacy can also preserve language, if the culture concerned has a strong tradition of storytellers, griots or bards memorising poetry or oral history. It seems that the coming of literacy has often meant the decline of the great feats of memory that people can accomplish when they have to.
Political continuity also tends to limit language drift. English lost most of its inflections after the Norman conquest. This was probably a combination of the effect of the reduction of the amount being written in English (scholars continued to write in Latin, but the language of law and administration changed from Anglo-Saxon to Norman French) and the effect of loss of status; English was seen as the language of peasants so nobody fussed about it being used "improperly". Another political factor is simply the use of laws to repress dialects seen as damaging to national unity.
Use as a language of religion or scripture certainly preserves a language. For example Latin, Sanskrit and Hebrew. On the other hand this only really works if the scriptural language ceases to be the language spoken in the streets. Classical Arabic is preserved in the Koran but spoken Arabic has diverged into very different forms across the Arab world.
(Update: After further research I replaced the original mention of Thai with Tamil. Greek and Aramaic are also examples of classical languages that can be read "naturally" by modern speakers. Hebrew is another example, but it was not continuously a living language. It is difficult to pinpoint when enough drift has occurred in a language to make it a "new" language, so there is controversy over which of these languages is oldest.)
