The math involved in this question is actually a lot more complicated than you might guess at first glance, and involves variables that only Disney has the data to fill.
At first glance you might be tempted to think that TSMM has a capacity of 1000 riders per hour, so a 90 minute wait would mean there are 1500 in the standby line. This would mean if you increased the capacity by 50% to 1500 then the wait would drop to an hour. This does not take into account the FP+ system though.
In reality no one outside of Disney knows what percentage of TSMM capacity is given to the FP+ system, but conservative estimates have placed it over 75%. That would mean only 250 riders can currently ride per hour through the standby line, so a 90 minute wait would represent 375 riders waiting in the standby line. This would mean if you increased capacity by 50% to 1500 and didn't increase the FP+ allocation at all, TSMM would be a walk on ride. Of course that assumes no one ( or less than 125 people per hour) who are currently scared away by the 90 minute wait would decide to ride or ride again.
I am going to guess that after a month or two of the new track in operation you might expect to see wait times ranging from 20-45 minutes depending on season and time of day. and perhaps much shorter in the early morning and late evening.
I would also expect as more new rides and experiences come online at DHS that you are going to see wait times drop even more. I would expect under 30 minutes once toy story land opens and probably under 20 once SW land opens