Each situation can be a little different, so it depends.
DLA can refer to the last payment, it could also depend on the acknowledgment (in writing), it can be a right of offset (for CIBC and TD account), and it can also mean when the revolving status changes on your bureau.
Once an account is established as an R9, the creditor will no longer update the "amount past due" column, as the full amount will be owing.
An account (when first assigned to third party collections) is usually an R5 (meaning up to 90 days past due) so, the DLA can change each time the R status changes (ie: R1, 2, 3, 4, 5, & 9).
It's a safe bet that the DLA stated on your bureau is accurate. I've heard of some circumstances (even on this site) where the creditor updated the DLA each time the account gets reassigned to a different collection agency.. that doesn't happen that often.
SO... if your DLA is after 2002 (I believe.. either 04 or 02) it's 2 years. Before it, it's 6.