I'm in a similar situation, but have several container plants that will require watering occasionally. Looks to me like it can be solved with a timer and a little work with hose or tubing for around $40.00. Check your local box stores.

Moisture retention granules are great for retaining water and then slowly releasing the moisture back into the soil as it dries out. Place the granules about 2" below the surface so that the water is released to the roots where it is needed. When the granules swell with water they look like clumps of white mini tapioca pearls or little white eggs on top of the soil. Remember you put this in the soil. This can be found in any garden center. I got mine from Michael's.

For the water alcoholics i use the

Drip-It Pro Waterer it keeps the soils way moist. i attached a link to where i bought mine. lol b/f anyone asks no i have no interest in this company nor do i get commission. i do though love their products & service.

http://www.gardeners.com/Drip-It-Waterer/default/17519.prd

I've used these the last few summers and they really work and aren't that expensive. I do try to move the containers into some shade if I can, and long windowboxes get 2 or 3 spaced out along the length, as do large round pots.

Here is a link that might be useful: watering cones

At big box stores I've seen something that looks like a plastic drink bottle that you fill with water and invert into the planter box. If I used my own bottles would this work?

No, they tend to let the water out rapidly. They are great for watering something over the course of an hour or so without worrying about top watering the soil and waiting for it to soak up.

The cones I have do work with regular soda bottles, but they have 'punch outs' that are poked out with a nail to create two tiny holes. The instructions indicate to fill the cone with sand as well though I don't really see the point.

Anyway, there is simply no good way to control the rate at which the water comes out.

I think Jeannie's link is a better solution (haven't used it, it just seems a more appropriate design for extended container watering) while the cones you speak of are a good choice for watering landscape plants.

Thanks for the suggestions. The product jeannejeanie mentions looks like a winner.

or the Adjustable Reservoirs:

http://www.gardeners.com/Adjustable-Reservoir/default/StandardCatalog.PotsPlanters_SelfWateringPlanters.34-507RS.cpd

There are two other low cost ways to water using capillary action. One is to wick water from the top and the other way is subirrigation from the bottom.

Materials:
1)A wick, a long segment of a nylon cord or a long strip made of super water absorbent fabric (even long strips of cotton t-shirt for short-term use).
2) Several bowls or jars to serve as individual bottom water reservoirs for each planter or one large container to serve as a community water reservoir large enough to hold water for the number of days you'll be gone. You'll need to experiment and do some trial runs obviously to find the right water reservoir size. The water container/bowl/jar/bucket size will depending on the days you'll be absent, types of plants, and the weather. A well fitted bottom water resevoir can be made of any recycled material and containers also.

OPTION 1: Thread a wick through one drainage hole and out another drainage hole of the potte plant to be watered so the wick ends are dangling and long enough to be soaking in the water in the reservoir below. Place and rest your potted plant with the wick on top of a container of water (well-fitted is preferred to keep water clean and minimize water lost). This water reservoir can be made of a recycled egg salad plastic container with or w/o the cover, the bottom of a soda bottle, and a larger bowl and etc to cover the days you'll be absent.

OPTION 2: Insert one end of wick into the soil surface (from the top) securely and the other end of the wick goes all the way into the bottom of a community water resevoir that is placed higher up (on a shelf or railing) than the pots being watered. For this system to wick water efficiently, the water surface on the reservoir has to be higher than the pots being watered.

Search for "self-watering planter" on youtube.com or video.google.com and you'll get some good visuals.

Good luck!

A temporary self-watering container may be easily created by stacking your potted window box inside another plastic window box if you happen to have a spare around or want to buy another window box.