Unpaid proposal work before a client says yes
Clarifying an assignment and producing work for a potential client can ask very different amounts of your time.

Decide how much unpaid proposal work to do by what you need to learn before quoting, what you would be producing for the client and how much time you can choose to invest. If the request includes work the client could use as a deliverable, agree how to handle that work before producing it.
What does the client need to decide?
A proposal needs enough information for both sides to understand the assignment and decide whether to work together. Ask which question the client needs answered before they can make that decision.
They may need to understand your approach, see a relevant example or clarify what the fee would cover. An existing sample you are allowed to share, or a discussion of how you would approach their situation, may answer that question. You still need to find out whether it does.
A request for a tailored analysis or a complete plan goes further. It asks you to produce something new for this client's situation. Name that output and estimate the work it would take before treating it as part of writing the proposal.
How much time can you invest before agreement?
Set a limit based on the time available in your business and what you need to learn from this opportunity. There is no single number of free hours that makes sense for every assignment.
Put the preparation alongside the work you have already promised and the time you need to find other clients. If you choose to spend an afternoon developing this proposal, decide where that afternoon will come from. A quiet week and a week full of delivery commitments give you different room to make the choice.

Write down what you will do within that limit. You might have a scope conversation, review the material needed to estimate the job and prepare a proposal. If new requests arrive, compare them with that plan before continuing. The fact that you have already invested time does not tell you whether the next piece of work is worth doing.
What can you offer instead of a full unpaid plan?
Offer a next step that answers the client's decision question without quietly promising a larger piece of work. Where substantial new work is needed to define the assignment, you can propose a separately agreed paid discovery step.
In a hypothetical example, an independent facilitator is asked to send a complete workshop agenda built around the client's internal problems before the client decides whether to hire them. Preparing that agenda would require reviewing background material and designing the exercises. The facilitator has enough information to explain their approach, but not to design the session responsibly.
They could offer a conversation about the workshop's purpose and show a non-confidential example of how they have structured a relevant session. That may give the client enough information to decide on the full assignment. It is worth asking which uncertainty would remain after that conversation.
If the client needs a tailored design before deciding how to proceed, the facilitator could propose a paid first stage to review the material and produce that design. Both sides would need to agree the output, fee and timing. They should also be clear about what the client will receive if they do not go ahead with the workshop.

The client may decline that first stage. The facilitator can then decide whether to invest more unpaid time within a limit they are willing to accept, change the offer or leave the opportunity. A smaller step only helps if it answers a question the client actually has.
What belongs in the project estimate?
The estimate needs to include the work required to deliver the assignment, including preparation that has not yet been done. Keep track of the time already spent pursuing the opportunity as well, so you can judge the whole effort it has taken to win and deliver the work.
Avoid counting the same preparation twice. In the hypothetical workshop, material reviewed during a paid first stage may reduce what remains before delivery. Check which parts can be reused and which still need doing when you price the full project.
When the proposal becomes an assignment, write down the agreed scope and what happens next. Later requests belong in a discussion about additional work, where you can agree any change before taking it on.
If several opportunities have consumed more unpaid work than you intended, bring the requests and the time you spent to a coaching conversation. We can look at the offers you are making, the clients you want and how that work fits into your week. You decide what to change in the next proposal.