When do SaaS Startups get M&A Offers?

There are many points along the SaaS startup journey where its ripe for M&A from acquirer perspective:

Is their a right time to be eaten?

• Product Market Fit (<$5M in ARR): Your startup is in a new or adjacent area for us (the acquirer) and you have proven product market fit. At this point, we can visualize you growing into a $100 to $200M business but you are young enough that we can swallow your tech, rebuild it as needed and go to market. This can be very lucrative for founders even if VCs won't like it as the total $ returned can be small. RelateIQ or Assistly/Desk acquistions are a great example here.
• Pre IPO ($30 to $150M in ARR): At this stage, you have taken out both product & market risk - we would buy you so you can be an entire new Business Unit for the company. Typically there is minimal rewrite - there may be integrations built to core. These became hard to pull of in 2011-14 because of the very high multiples private companies had but may again be happening soon as public markets are ruthlessly compressing multiples at IPO. If SAP were to buy Docusign right now in 2015 end, it would be a great example.
• Post IPO: This is common and well understood. Oracle bought Responsys, Salesforce bought ExactTarget. Easier to negotiate these transactions as the market price is easy to verify. Hard to do in real world because SaaS businesses are hard to integrate without a lot of rewriting. This is why Aneel Bhusri and others have stated - in SaaS you have to build to be truly a winner. 
• Team Sale: This is a failed startup having a hard time raising money typically after angel round but can be any stage. You sell for whatever you can make. This is one area where top tier VCs are often good - they have connections and can make a soft landing lucrative. Some have argued Diane Greene's startup acquired by Google fits this pattern.