How many hours needed to train for HIM?

There is no stock answer as it is going to vary greatly based on current fitness and goals for HIM. 

For me,  I would want 10-15+ hrs

To get a good idea, go look at the Beginner HIM plan from here.  That will give you a good idea of the hours... that plan is targeted towards those who want to "just finish."  It is around 7-10 hours per week.

Good luck! 

The exact amount of time depends on your goal, your facility at each sport, the time you have to spend training and your dedication to achieving your goal.  If you are a novice at one or more of the sports, you may have to train longer.  But if, for instance, you can already bike 180 kms and run 42.2 kms then you have to train to do bricks.  You would also have to train to swim a long distance without tiring.

I was pressed for time also before I retired also, what with all sorts of commitments.  But I managed to squeeze in my training by building it around my working schedule.  I biked to work early, did laps 3 morning per week, ran at lunch 3 times per week and did yoga at lunch 2 times per week.   I did the long bike and run training early in the mornings on the weekend.   

The answer of course is, it depends.

Most plans that are designed just to get you to the finish line are in the 7-10 hour range, intermediate plans are generally in the 10-13 hour range and if you want to place, more.

I have my 3rd HIM this weekend, and most of my weeks has been in the 9-12 hour range for all 3 HIMs.  I am hoping to move up from the BOP to the MOP.

Assuming that the tri you're looking to do in 2:40-2:45 is an oly, you won't need to put in much more training than you're already doing. The swim in a HIM is only slightly longer so your current swim training should be sufficient. You'll need to build your long runs up to about 14 miles and your long rides up to about 65 miles so that may add another hour or two per week than you're already doing.

I did my first HIM last year in June. My training consisted of doing a total of maybe 6-8 swims in the 6 months prior to the race, and maybe 8-10 bike rides that included 2 of 60+ miles. I had trained for a marathon that I completed a month before the HIM, but my training was very minimal there as well. Despite such little training I still finished the HIM in less than 6 hours. I was 51 then, so at your age you should be able to do even better.

One thing to keep in mind, having a new child will completely change your life.