Depends upon what you're selling and how much it costs. Frequently, larger companies have a set budget limit at certain levels where one person cannot make the ultimate decision about buying so a committee needs to be formed which draws out the cycle. In smaller companies, the owner or CEO makes the decision and doesn't need anyone else's approval so the cycle is much shorter.
If you're selling a complicated technical item (such as a large phone system/PBX), the cycle will be long due to the impact on the organization. There are too many areas of the business being impacted which will require training and implementation coordination so the decision cannot be made quickly. A new computer system and/or network would be similar. If you're selling something like paper products, it might not take very long to reach a decision because the organization isn't impacted that greatly.
So the shorter answer is it depends on:
1) Item/solution being sold
2) Cost of the item/solution
3) Impact on the organization (complexity)
4) Number of decision makers required
5) Time to implement & train staff
6) Ongoing support
Once you have uncovered the answers to each of these items, you should have a good idea how long it might take. Additionally, it may take some time to acquire all of this information to share with your organization in order to customize the proposal.
That's a loaded question, but I hope I answered it somewhat.
