In a slip and fall must I prove how long the piece of watermellon was on the floor

This is a Slip & Fall transient substance case and if you can't meet the generally impossible burden to prove the store created the dangerous condition or knew of the condion, then you must prove constructive notice: that the dangerous condition existed for a period of time greater than was reasonable under the circumstances: ie. generally 15 minutes will be sufficient. Get some help. Slip and fall cases are difficult even for experienced attorneys. Over the last 30 years we have handled and resolved a wide range of approximately 1,000 Slip & Fall cases.

Correct...the Plaintiff has the burden of showing that the store was on "notice' (actual or constructive) of the piece of watermelon on the floor. If store video doesnt exist , your counsel can retain an expert to examine the photograph and opine on how long the food was on the floor. If you dont have counsel, then retain one for your case. Good luck.