Having submissions open for too long leads to inaction on the behalf of the candidate (no pressure to submit now). This may put a dent in your marketing budget and have very little outcome. Yet, taking submissions over too short a period could be detrimental to the turnout (candidates may feel they don’t have time to apply).
If you’re not asking candidates to create something for you specifically; we’ve found opening submissions for 6 to 8 weeks to be the perfect length.
If you are asking applicants to create something new for you, then the length that you open submissions for should reflect the effort you expect from them. Give them the time needed to work on your brief. Unless submissions are for a competition or an educational context, it’s worth noting that asking for bespoke work from candidates without some form of payment (spec work), will lead to fewer submissions of varying quality. Consider alternative ways of making a call out.