Links to assets shared within BAM! are secure and accessible within the BAM Viewer by anyone with that link, until the link expires. The expiration time of those links is determined at the user level when the asset is shared, and administrators can set a minimum and maximum number of days available to the user.
As an administrator, asset sharing duration can be set from within General DAM Settings under the Settings tab:
Assigning the minimum and maximum allowed share days to the same value will assign all shared links that same static value, and the user will not be prompted to enter a duration when sharing assets: