I've sold 4 startups. Median time from starting the process to close was 6 months, with the quickest transaction taking 2.5 months and the slowest almost 9 months.

Diligence ranged from 3 weeks to 7 weeks.

Fastest transactions were private company acquisitions. Slowest were public company deals.

From the time we decided it was time to sell (for real, not just theoretically) until the money was in the bank, almost 18 months. It took us 2 months to find an advisor, then another 3-4 months building the deck and investor material (cashflows, projections, etc). We put up a bid, so we had some 8 companies interested at first and we narrowed that down to 3. That process was LONG, but that is what gets better valuation - competition. After we signed the term sheet it was another 6 months due to regulation and legal quirks, due to the acquirer being a public company. Those were the longest 6 months of our lives :)