It depends entirely on the donor's records. A relative with organised tax returns and a single bank may take weeks; a family business spanning decades and two countries can take far longer. Build the schedule around the slowest document, and do not commit to a subscription deadline before the donor's evidence is in hand.
Give the donor a dated request list
Send one written list with deadlines rather than repeated ad hoc requests, and review progress every two weeks so a stalled item is visible. Where a document must come from a foreign bank or registry, start that request first. After the petition is filed, the pace is governed by the government and by visa availability, neither of which can be promised, so avoid arrangements that depend on a decision arriving by a particular month.
Add one further discipline to that fortnightly review: record what has been asked and when, so a stalled item is visible as a date rather than as a feeling that things are slow. Where a document must come from a foreign bank, a registry, or an institution that has since merged, note the request date, the expected response time, and the alternative if it does not arrive, and treat the absence of a reply as a decision point rather than a delay to be absorbed. Hypothetical example: a request to a foreign bank for archived statements sits unanswered for seven weeks, and because the log made the silence visible, an alternative source is identified in week eight rather than in month five when the rest of the file is otherwise complete.