Most first time harp makers or players are a bit shocked to learn that tuning up their harp for the first time requires a significant amount to time spent tuning their instrument. And, this not only applies to harps but to any stringed instrument.
What I mean is musical instrument strings, depending on the string material used (metal, natural gut, nylon, etc.), will resist expanding or more likely for nylon, will loosen & stretch excessively each time they are tightened. However and thankfully, over time during frequent tuning, the string will eventually keep its stretch & subsequent pitch (i.e., 'tuning'). The tighter the string is pulled, the higher the pitch the string will play (towards the treble notes).
What makes a harp unique, though similar to a larger lyre is we harpists have so many more strings to tune! (Or, consider the work a piano tuner must do to keep their piano in tune!)
Most find that we have to tune new harp strings 2 or 3 times a day for a week or two in order for the harp to quickly become closer to tuning stabilization (i.e. not having to tune it so often). By the end of the 3rd week, the harp should be getting close to not needing tuning but once a day. By the end of the month, tuning every time you play would be all that is necessary.
I've found, from about 6 months through the first year with proper, regular tuning, I don't need to tune each harp so often. Yet, on the other hand, the more often you play, the more the tuning will become necessary as the strings are constantly being stretched and thus, detuned when played. Plus, nylon harp strings do wear out eventually and will need to be replaced. See our harp accessories page for new strings for your harp.
And, add the fact the harp's Soundboard will also flex (to whatever degree while under the great pressure of so many strings pulling up on it, depending on the design of your particular harp). This too, will work against the harp's stabilized tuning over the first year but eventually will stabilize.
The saving grace for modern harpers is the electronic 'e-Tuner' we now use for tuning our many harp strings! If you need one, we have them here. Also, see my Awesome Links page for various harp resources links including an on-line Middle C tone source.