I am currently topping up my coolant level from min to max approx every
3 - 4 months (I use a 50 - 50 antifreeze/water mix). During this time I am probably covering about 3500 miles. The vehicle has done 137k.

Is this normally acceptable or should I be getting this checked out.

cheers

I wouldnt worry too much about that much loss over that mileage or time period.
Your mileage is high and you may have a slight weep on a hose or even the normal craked header bottle (round the cap) which will cause some to leak but dont panic.