What is the validity period of an encumbrance certificate?

An encumbrance certificate reflects the registered transactions on a property up to the date it was issued — it isn't really something that "expires" in a formal sense, but it also doesn't reflect anything that might have happened after the date it was generated, so an older certificate simply becomes less useful the more time passes.

Since your deal is taking longer to close, it would be prudent to obtain a fresh encumbrance certificate closer to the actual date of registration, to confirm nothing new — like a fresh mortgage or another sale attempt by the seller — has been registered against the property in the intervening period.

This is a relatively simple, inexpensive step to take given how much protection it offers against last-minute surprises, and most property lawyers routinely recommend getting an updated certificate right before final registration as a standard precaution.