It depends on the state. In most states you have to purchase comprehensive coverage that has Full Windshield or Full Glass Coverage in the deductible section. Some states don't offer this option at all.
For example, in Florida windshield coverage is included when comprehensive coverage is purchased (regardless of the deductible option selected) due to state laws that provide with comprehensive coverage your windshield repair or replacement comes with no deductible.
Under most circumstances your front windshield is covered if you have full coverage. Typically, the term full coverage includes comprehensive and collision.
With full coverage, in some states you do not have a deductible for the front windshield, but all other windows are subject to the deductible that is why you have to read through your policy and speak to your agent if you have questions about your coverages.
If you only have liability insurance on your vehicle, you will have to pay out-of-pocket for the replacement of your windshield, since it is not covered by your policy. Liability coverage only covers others that you harm, not your car for any damages it receives.
Comment Update: Your math of $250 + $350 = $600. The easiest way to understand the deductible it this, in the event of a loss you are responsible for the FIRST $250 (the deductible) and the policy will pay the rest.