You have taken the responsible step of insuring your prized possession - is it enough? There is no easy answer to this question. It's not like insuring your diamond ring which can be appraised and a specific value set, nor is it like insuring your car where the company has tools to know how much the car cost new and how much it has depreciated to help determine market value after it's been totaled.
Equine mortality is designed to help you replace your horse should the unthinkable happen. What is the value you are comfortable with going out to shop for a replacement? Do you want to go out and shop for a prospect that you need to build back up to the skill set your horse has? Or are you hoping to stumble across that super steal of someone who needs to sell their well trained horse? Or do you want to be able to have the funds to go out and shop for a horse of similar talent & ability?
While there is officially no top limit at which you can insure your horse, the carrier is only going to approve to market value. If you just bought your horse (even one that was a super steal and is worth at least double what you paid), the price you paid is what sets the current market value. For a horse you've raised or have had for awhile, other factors help to determine value. Unfortunately the cost to feed or board them doesn't increase the value, but the cost of training or lessons does. Also, if you have accumulated any breed show points or won futurity money, or even just gone to the open show and taken home a bunch of blue ribbons - these things all help to increase value. I tell my customers that when we are setting the value after purchase (or even a horse the customer has raised) we are basically "selling" the underwriter on why the horse is worth $XX value. This can include information on half siblings or horses of similar breeding and talent (with available documentation). There is a form we fill out to help substantiate value and I like to treat it like you would a sales ad - "here's all the wonderful things about my horse and why you should write me a check for the amount I'm requesting." You can also hire an official equine appraiser to help determine value.
I have some customers who have horses that are worth $20,000 but they are comfortable for insuring for $10,000 to help them go shopping for another. Others want to insure for the cost it would be to replace the horse. There is no right or wrong answer - it's insuring for the limit you are comfortable with having and paying for. So have this discussion with yourself - the best time is at the renewal of the policy but a value increase can be requested anytime during the policy year as well if your horse is having a banner year.