Are you buying and using pre-terminated (RJ-45 connectors) Ethernet patch cables or are you buying lengths of cable and installing your own RJ-45 connectors?
Personally, I prefer to buy my own cable and connectors and terminate the cables myself. There's little waste, the holes you drill are smaller and you don't have to baby and coddle those RJ-45's that are already on pre-terminated cables to keep from breaking the latch....by the time I've got some paper (to protect from tape glue) then tape over the connector, it looks like I'm trying to pull a roasted chicken thru a hole just barely big enough for a hot dog.
You can also buy 2 each 250' or 500' boxes and pull 2 cables at the same time; or 4 each 250' boxes and pull 4 at a time...it's your call. Sure the cost/foot goes up when you compare a 250' box to a 1,000' reel but chances are you're doing this by yourself anyway and to me, time is money. I'd rather spend $100 (instead of $50) and save an hour (also worth $50, IMO) and save my aching back, also....which is hard to attach a price to!
I have found that if I estimate a job will take 250' of cable I buy 500', especially if I know I'll likely need more in the future on another job. Nothing more annoying to me than having to stop, drive 25 miles each-way (for me, at least) to buy a $4 box or $5 worth of connectors; I over-buy and if local, I can return for refund at MY convenience.
True, it's hard to return items purchased online but it's a coin toss....I prefer to have too much than too little, once I start a job I want to finish it. At 69 it's hard to get started, I like to keep what little momentum I have going.
Generally, with quality, all copper CAT-6 you can run POE up to 328 feet total length. You can extend an existing pre-terminated cable with a female-female RJ-45 coupler but I DO NOT RECOMMEND THAT. I have done it in a protected, indoor environment as a temporary measure until I can replace the cable soon. The more mechanical connections you introduce into any circuit, the more possibility of problems. Extreme temperature swings, especially in an attic, can introduce thermal intermittence with connections, especially something that depends on spring pressure and friction to make and maintain a good, low impedance connection. You don't need that. For each camera use 1 cable with a RJ-45 at each end; 1 at cam, 1 at NVR.
