Here is 6 months with a tiny amount of fuel dropped as well.
Kvikk class Missile Boat 5,800 tons 167 Crew 818.4 BP TCS 116 TH 480 EM 0
4138 km/s Armour 3-28 Shields 0-0 HTK 49 Sensors 0/0/0/0 DCR 1-1 PPV 24
Maint Life 1.44 Years MSP 588 AFR 269% IFR 3.7% 1YR 312 5YR 4,679 Max Repair 240 MSP
Magazine 272 / 0
Captain Control Rating 1 BRG
Intended Deployment Time: 6 months Morale Check Required
Orobos Turbines NPE-EP480M (1) Power 480 Fuel Use 110.23% Signature 480 Explosion 15%
Fuel Capacity 255,000 Litres Range 7.2 billion km (20 days at full power)
Temetor Ordnance S6ML (4) Missile Size: 6 Rate of Fire 25
Temetor Ordnance MFC49-R20S (1) Range 49m km Resolution 20
Temetor Ordnance ASM6 Mk2 (45) Speed: 19,200 km/s End: 4.1m Range: 4.7m km WH: 9.00 Size: 6.00 TH: 64/38/19
Vivar Warning & Control AS52K-R50 (1) GPS 4000 Range 52.6m km Resolution 50
Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s
This design is classed as a Military Vessel for maintenance purposes
This design is classed as a Warship for auto-assignment purposes
If you are thinking just simple early game planetary defense. This doesn't seem too bad. Personally I'd add some more armor. 3 is really low for a ship that'll see combat. I like to do at least 5 or 6 and more as space permits.
I'm not sure what your missile design looks like so it's tough to say how hard it'll be to hit. Do you have decoys enabled for instance. If you don't have "smart" missiles then larger volleys are needed. 4 isn't much by itself, but if you make say 10 of these ships, now you have a pretty good sized volley.
Lastly, I highly recommend creating a second ship with anti-missile duty. I've had some games where the opfor shot missiles at my colony, not my ships. Bigger target I guess... And you'll want to be able to stop those. You can use STO ground forces for this if you like. Railguns or gauss canons.