When you sell your SaaS Offering in a Try/Buy model, picking the right length of the trial period is key.
The goal of a Try/Buy model is to let your customers experience the complete value proposition from day 1 (as opposed to a freemium/premium model). Hence the trial should focus on:
Fast adoption of using the key capabilities
Experiencing the Value of the Product to warrant buying it
For most B2B Solutions this will include an on-boarding effort of the user. Hence the best way to think about the length of the ideal trial is the sum of:
Length of reasonable sign-up/activation and onboarding to use the full service
The time it takes for the customer to accrue measurable value from your service
It goes without saying that the trial period needs to be serviced with great follow up through nurture and onboarding emails, Customer Success support (calls, chat, whatever fits your solution).
Don't make the trial period too short so it does not allow for the experience of value. Don't make it too long and leave money on the table. Keep tuning it as you go.