Synaptic strength can be defined
»as the average amount of current or voltage excursion produced in the postsynaptic neuron by an action potential in the presynaptic neuron«
Synaptic strengths and their changes depend on many factors and mechanisms, from LTP and LTD to STDP and STF and STD. Possibly, also neuromodulators should belong to this list – but maybe they shouldn't.
Assuming that synaptic strength is a well-defined quantity even when the synapse is at rest, the strength of a synapse is a continuous or non-continuous function over time, that ideally can be plotted.
Now I wonder whether there have been recordings and/or simulations of the synaptic strength of a single synapse as such a function over time - and how the plot would look like at different time scales: long term trends superimposed by shorter and shorter fluctuations?