Let us first clarify the relevant terminology. There are different types of arevim, with the two main ones being a regular arev and an arev kablan (literally, a receiving arev). An arev kablan – who attains that status by means of explicit agreement or sometimes even through more subtle language and/or circumstances – is treated like a direct recipient of the loan, even though the practical beneficiary is someone else. This type of guarantor has a heightened level of obligation. While a lender can demand payment from a regular arev only if the borrower does not have enough money or assets from which to pay back the owed money,[1] a lender can demand payment from an arev kablan even if the borrower is capable of paying.[2]
The Rambam[3] writes that an arev is not required to pay until 30 days have passed from the time he became obligated, just as a borrower is afforded 30 days. The Bach[4] understands this to mean that the arev is treated exactly like the borrower for whom he is covering. Therefore, just as the borrower’s 30-day grace period after the time he is obligated to pay applies only if he does not have liquid assets to pay immediately,[5] the arev is granted 30 days only under parallel circumstances. The Shach,[6] however, argues cogently that it is evident from the Rambam that the arev has an advantage over the borrower. Even if the borrower’s time to pay has already passed, the arev’s 30-day clock starts ticking only at the point the payment of the debt becomes his responsibility. Just as a borrower generally has use of the loan money for 30 days until the obligation to pay kicks in, an arev has 30 days to get around to paying, even if the borrower’s obligation has already started and the arev is already now capable of paying.
However, it would seem that the Shach’s ruling should apply specifically to a regular arev; it does not seem relevant to an arev kablan. Since the arev kablan accepts the responsibilities of the borrower, he does not deserve extra time beyond that which the borrower is granted. In the practical realm, since the lender can extract payment from the arev kablan without first trying to do so from the borrower, the arev kablan should not be surprised when the payment falls on him, and he should have planned his payment as the loan was becoming due.
However, we must still ask: Even if an arev kablan is like a borrower, is a borrower himself in fact supposed to pay as soon as his loan is due? The gemara[7] states that “the time of beit din is 30 days.” This means that from the time that beit din rules that an award should be paid, the standard time for carrying out the court’s ruling is 30 days. Thus, if one admits to beit din that he owes money but requests time to raise the amount, beit din gives him 30 days to raise it.[8] The Rama[9] writes that this is only a default period of time. If the beit din understands that the litigant needs either more or less time, they should adjust the 30-day length accordingly, including by requiring immediate payment for those with the wherewithal.
The same is true of your case of arev kablan. In theory, he should pay right away; in practice, however, his request to push off the payment for 30 days (or a different time period, as per the Rama above) will be accepted when based on legitimate need.
The Shulchan Aruch[10] cites two opinions regarding whether a borrower can request a 30-day grace period for payment when the target date of payment was set in advance. The rationale of the stricter opinion is that the borrower had preparation time for the payment, and he should have completed the preparation by the stated time.[11] There is strong logic to argue that in the case of an arev kablan, all would agree to give him the 30 days upon a reasonable request, even if the loan had a clear payment date. This is because even though the lender may come to the arev before the borrower, it is by no means clear that he will do so. Thus, the arev kablan can legitimately argue that he needs the normal amount of time to prepare payment, even according to the stricter opinion.
In summary, an arev kablan should pay the debt as soon after the lender’s request as he reasonably can, which may often be immediately. If there are difficulties, the standard grace period is 30 days from the time he is asked to pay, just as it is for a borrower. It is possible that our assumptions about the reasonable expected speed of the payment may be slightly different for an arev kablan than for a borrower.