Earlier than seasonal work usually gets planned. The supporting professional's description and the employer letter are the slow parts, and both depend on people whose priority is the project rather than the paperwork.
Work backwards from the season, not forwards from today
Field and laboratory seasons have fixed start dates, and the temptation is to leave the paperwork until the work is confirmed. The difficulty is that confirmation often arrives a few weeks before the season, which is exactly when the supervising professional is least available to write anything. Ask for the technical description while the planning is happening, even in draft, and revise it later if the projects change.
Set your own checkpoint several weeks before the intended start: if the employer letter and the professional's description are not in hand, the start date should be discussed rather than assumed. That conversation is far easier in advance than at a port of entry. Hypothetical example: Darnell is needed to assist a hydrologist with a flood-model validation project that begins after spring runoff.
A timing review should work backward from the project start: first secure a precise technical description, then collect education and method records, and finally choose the appropriate admission or petition sequence. The project plan and the supervising professional's availability are the records that usually set the real deadline. Do not treat a fast operational need as proof that the application is ready.
Delays commonly occur because the employer writes a general offer first and only later tries to identify the scientific professional and the specific work being supported.