Similarity Threshold Standards
I don't think there's any standard, but there might be some exceptions in very specific cases where the distribution of the scores is known precisely.
There's no standard because in general the optimal value of the threshold strongly depends on the task and the data. That's why thresholds are usually determined empirically based on the desired outcome. In other words, a threshold can be seen as an hyper-parameter: its optimal value can be found by maximizing the performance of the target task on a training set (or validation set).