If it doesn't state, then the vendor is at a disadvantage as the purchaser can take his own sweet time to apply for loans with several banks and slowly decide as to which to choose before signing the SPA. In the law of contracts, market practice does not apply to vary/amend/insert any terms of the letter of offer. What matters is what was agreed and executed upon by both parties.
What needs to be determined is if the letter of offer is such that it forms a binding contract between the parties.
Two scenarios:
1) If the purchaser has paid 2% deposit to the vendor, then there is a binding contract as consideration (via the 2%) has moved from the purchaser. The letter of offer is a binding contract and parties are bound by its terms. If the vendor does not stipulate a fixed timeframe for the purchaser to finalise and execute a SPA, it can be inferred that the time given is indefinite. As scary as it may sound, in reality, it would not drag too long as surely the purchaser would want to settle in his new home as well.
2) If the purchaser has not paid any deposit to the vendor but merely executes a letter of offer, then it can be argued that there is no binding contract as no consideration of any kind moved from the purchaser to the vendor. All the purchaser did was offer to purchase, and the vendor accepted, but nothing was given to the vendor to 'bind' the vendor to honour that acceptance of the purchaser's offer.
In scenario (2), the vendor can unilaterally terminate the letter of offer. However, it would be prudent to first give the purchaser a timeframe (even if the letter of offer does not stipulate one) - perhaps a letter from the vendor's solicitor to the purchaser (or his solicitor), giving him a further 7 days to prepare and send a draft SPA to the vendor's solicitors, failing which the letter of offer would be terminated and be of no further effect.
Another situation where no consideration has passed is where the 2% was given to the purchaser's solicitors as stakeholder pending approval of purchaser's loan. If the loan is not approved, all of the 2% would be returned to the purchaser. Once again, I am of the opinion that this would fall under scenario (2), as no real consideration has passed to the vendor. If the vendor has the right to forfeit the 2%, then yes, there is some consideration. Otherwise, there is nothing to bind the vendor.
Consideration does not need to be adequate, it just need to be sufficient. In other words, it doesn't need to be 2%. Even if it is RM1.00 from the purchaser, which the vendor accepts, it is enough to form a binding contract, as sufficient consideration has moved from the purchaser to the vendor.
Hope the above helps.

Thanks Dario as always for the detailed advice...