A best French toast is supposed to be slightly crisp on the outside yet creamy - custard like on the inside. The reason why it would turn soggy could be due to the following reasons:

1. The Bread is not thick enough:
The ideal thickness is between 1/2 inch to 1 inch.

2. Wrong kind of Bread/ Fresh Bread:
Check out my French Toast Recipe for the type of breads that are excellent for making French toast. I mention them right in the beginning of the post. Stale bread or bread that sits on the counter for few hours/overnight before making the toast works better than fresh bread. Fresh bread gets soggy in few seconds of dipping into the custard.

3. Diary imbalance in the custard:
Adding too much milk to the custard will result in uncooked soggy French Toast.

4. Using too much heat:
The sugar content in the custard will caramelize quicker in high heat without cooking the insides resulting in burnt yet soggy toast. Medium - High heat is the ideal temperature with 2-3 minutes of cooking per side of the toast.